I wrote few recommendations for my friend this summer. Parts on train and bus schedules are relevant only for past summer, other info may be useful to someone.
Soča Valley
Place to go for water adventures.
In the valley there are three main cities: Bovec, Tolmin, Kobarid. All of them really small, I would suggest to stay in Bovec.
To get there from Ljubljana:
Train + bus:
Train from Ljubljana to Most na Soči (https://www.slo-zeleznice.si/sl/component/sz_timetable/?vs=42300&vi=&iz=44707&da=2019-06-11T00%3A00%3A00&showDa=11.06.2019#stationsData#stationsData)
Bus from Most na Soči to Bovec
Bus (Recommended):
https://www.soca-valley.com/mma/vozni-red-vr%C5%A0ic-kranjska-gorabovec-2019pdf/2019052810491399/
Goes twice a day, every day:
9:30 - 13:10 15:00 - 19:10
To go back to Ljubljana from Bovec, goes twice a day, every day:
9:30 - 13:17
15:30 - 19:17
Stay in https://hostelsocarocks.com
Hostel offers some activity packages. Zipline, rafting, canyoning, kayaking... I like all of them, zipline and canyoning especially , and you get really close to the nature. If you don't want to book through hostel I can recommend these guys: https://www.top.si/
There are great biking routes around Bovec. Maps are available in TIC in center of Bovec: https://www.soca-valley.com/en/soca-valley/about-soca-valley/tourist-information-centre-/2017113015444153/ Rent a bike shops are next to TIC. I like this route (Start and end in Bovec): https://www.soca-valley.com/en/summer/cycling/36197438/#dmdtab=oax-tab1 you can jump into the water at Slap Virje (waterfall Virje).
I like this part of Soca river: https://goo.gl/maps/1b2MGo96sVLn2hXi6 you can't do water sports on that part because it is too narrow, so you have to walk by the river. This is path from "Mala Korita Soče (meaning small Soca canyon)" to "Velika Korita Soče (meaning great Soca canyon)". At the end of "Velika korita" you can jump in: https://www.youtube.com/watch?v=omh3aHvuSRU
Beautiful easy (3h) hike to Krnsko jezero (lake Krn): https://www.soca-valley.com/en/attractions/nature/lakes/2018012510053404/ Trail starts here: https://goo.gl/maps/wWRCUpm1oRMNfTgw5 (25min with a car from Bovec) If you feel like hiking some more you can sleep there in a hut (20 minutes from lake): https://en.pzs.si/koce.php?pid=39 and make summit to Krn (3h) next morning and then head back to Bovec.
To get something to eat in that area I would recommend Hiša Polonka (House Polonka): https://goo.gl/maps/zkTuJA2whn9hN6ne8 . It is a side project from the most famous chef in Slovenia (Ana Roš - she was named best world female chef last year). They serve typical dishes of that area. If you want to spend more money, you can of course visit her main restaurant Hiša Franko, also in the area, but it is crazy expensive and reservation is needed.
Kranjska Gora
This is a ski resort in winter and in summer it offers great hiking.
To get there from Bovec or Ljubljana use the same bus mentioned above (it stops in Kranjska Gora where you have to change bus if going to Ljubljana/Bovec). You can also make a day trip from Ljubljana to Kranjska Gora (bus arrives at 11:30 and leaves at 17:30).
Rent a bike there and go on an easy route to Belopeška jezera (Belopeska lakes - there are 2) (they are actually in Italy but there is no real border). Its around 15km so you can make it there in less than an hour. You will pass nature reserve Zelenci shortly after start. Zelenci: https://www.youtube.com/watch?v=e-Dx4my1GKw Whole bike trip to Belopeska jezera: https://www.youtube.com/watch?v=6vljxpXnC5o Map: https://goo.gl/maps/w8DxfRM3AnNgr61n8
On the way back you can visit Planica, where one of the biggest ski flying hills is: https://www.nc-planica.si/en/activities/ if you stay for longer, you can go ziplining or try cross country skiing in underground tunnels.
Easy hike (1 hour up, 40min down) is from Planica to Dom v Tamarju: https://goo.gl/maps/vcZd8eiQnTZaxE5h9
Medium hike to Martuljek waterfalls ( https://www.kranjska-gora.si/en/sightseeing/natural-sights/waterfalls/martuljek-waterfalls) https://www.youtube.com/watch?v=v_lDJ6Ybe64
Bohinj
lots of buses every day, could be crowded/full (you can do a day trip): https://www.ap-ljubljana.si/vozni-red/?vstopid=1&vstopnaziv2=LJUBLJANA%20AVTOBUSNA%20POSTAJA&izstopid=569&izstopnaziv2=Bohinjsko%20jezero/lake&datumvstopa=11.06.2019&ponudnikid=0&rnd=03RUF
Bohinj lake is great for swimming, don't know how crowded it is right now
there is a cable car to the mountain Vogel - on the mountains is a bike park: http://www.vogel.si/summer/options/mountain-bike-park
hike to Slap Savica: https://www.bohinj.si/en/attractions/savica-waterfall/
a little bit before lake Bohinj is lake Bled, which is more touristic (it is the one with church island), if you decide to visit Bled, check out Vintgar Gorge http://www.bled.si/en/what-to-see/natural-sights/the-vintgar-gorge
if you feel like hiking go for a two night (three days) hike to see 7 lakes: http://jostgantar.com/2014/11/popoln-dan-v-dolini-sedmera-jezera/
The Sea and Caves
Slovenia is full of caves. Most well known is Postojna Cave, there is also a Predjama castle there (castle in a cave). You can do day trip there from Ljubljana. https://www.postojnska-jama.eu/en/
Bus from Ljubljana directly to the cave: https://www.postojnska-jama.eu/assets/Uploads/attachments/Avtobusni-vozni-red-zimski-2018.pdf
Not as touristic are Škocjanske jame (Škocjan caves). Best way to get there is with a train (a bus waits for you at the train station to get you to the caves) - https://www.slo-zeleznice.si/sl/component/sz_timetable/?vs=42300&vi=&iz=99996&da=2019-06-14T00%3A00%3A00&showDa=14.06.2019#stationsData#stationsData
On the other side of the country there are mines under Peca mountain. Mines are now closed, but you can go in there with a bike, kayak or a little train. Kind of hhard to get there though, public transport is rare and strange hours (late afternoon): https://www.podzemljepece.com/?lang=en
Ljubljana -> Postojna Cave -> Škocjan Caves -> Piran (Sea side) is 1 hour 40 minutes with a car: https://goo.gl/maps/r8XfRCxTGPcyxRWb7
Piran is the prettiest sea side town in Slovenia: https://www.youtube.com/watch?v=1wH_4IEO1yo you can do a day trip to Piran if you want.
Best beach is called Mesečev zaliv, only accessible by foot: https://goo.gl/maps/apvu9bEPyu5zYXa66 (1 hour walk from Piran). It is not spectacular though.
Goriška Brda
Food and wine. This region is full of wineries and small wines. It is hard to get there without a car, so if you want to see this place I would suggest to join a tour.
Don't know how long you want to stay, you can do day trips from Ljubljana (to Bled/Bohinj, Kranjska Gora, one of the caves and Piran, Brda). If you decide to go to Soča valley (Bovec), stay there longer (don't do a day trip).
You can also go Ljubljana->Kranjska Gora->Bovec->Ljubljana, and do a day trip from Ljubljana to Bled/Bohinj or caves.... you have lots of options :)
In Ljubljana:
best coffee is in Cafe Čokl https://goo.gl/maps/T49rCPKjc6QGvquR8
you can get cheap fresh fish under the main market (under the arcades beside the triple bridge): https://goo.gl/maps/Ys8sgaT15geuTzrv9 (freshest fish is on tuesday and saturday)
I like bar Magda at the main market (great selection of gin tonics): https://goo.gl/maps/fgHZnKSvDKBTDxiG7
there is open kitchen every friday here (from 10:00 - 21:00): https://goo.gl/maps/1DLuX6emqx7ULAtc6 lots of different restaurants cooking outside, you can try different foods
best burger: https://goo.gl/maps/eSbzNTeTqxFF1RTM9
nightlife, don't know what you like, lots of different clubs but pretty alternative place is metelkova:https://goo.gl/maps/p8eGDAofeby8M1uH6 clubs usually open around 22., maybe later
best breakfast: https://goo.gl/maps/uiddKzJC22LHLvni6
on a hot day try beer called Bevog Deetz, most of the pubs near the river should have it
Let me know when you come, if I'll be in Ljubljana we can grab a beer. Also, people (especially outside of Ljubljana) are really nice, so if you get lost or can't find anywhere to eat/drink just ask them, they will be happy to help you. If you decide to go into the mountains use good shoes and be careful - although paths are well maintained, accidents happen way to often.