Dude/Dudett above dropped some wisdom:
-I'd second the cheap airlines. Once COVID restrictions lift, you need to be traveling on the cheap EVERY opportunity you get. This is a once in a lifetime opportunity. The cheap airlines and AirBnB are your friend. Porto was my favorite. Greek Islands are incredible. Southern Spain....Europe is your playground. Go play. Also, give AirBnB Experiences a try. A great way to meet people and try new things in big cities.
-To find cheap airline tickets, I used Google Flights. Plug in your dates, and hit explore destinations from your originating city. I would go to wherever was cheap during that weekend. Saw all of Europe that way. Also look at nearby airports. Reserve parking ahead of time as it's not guaranteed. Pack lightly
-If you get into Alpine hiking always go with a buddy and someone who has experience (hiking groups are a good way to go). Check the weather. Check the difficulty level. I loved the Glecksteinhütte in Switzerland.
-Google Fi worked well for me (I kept it the whole time I was there) and kept my U.S. number. I did buy a Netto data-sim and used a hotspot for data for something cheap (10 Euro's a month) . Do NOT use TKS. They will rip you off.
-I never got a German bank account. Although they highly recommend it to pay speeding tickets, etc. Some places don't accept our credit cards (I'm looking at you Strasbourg unattended airport parking garage). In those instances, I found that GooglePay/Apple Pay was always there to bail me out. Also, carry cash.
-Find out your APO (or a co-worker's). Buy those $30 rubber shipping containers. Ship your nice stuff (electronics, photos) via USPS and get it ensured. Shipping up to 70 lbs is around $50 I believe.
-Before leaving, go get an International AAA driver's license.
-Never leave the house without a phone external battery charger. That is your lifeline (bus system, transportation, etc).
-If you've already been vaccinated, make sure you keep your proof of vaccination handy -- that will help you travel for the open countries.
-If you don't already have your blue civilian passport, go get it