Haha i did the same thing for my kids once. Twin boys so I had to make everything available for 2 players to do at once.
Here are the cool things I made:
A big old classic style farm, with farm tools and animals to play with. With plenty of animal feed in chests so the kids could have fun breeding cows or whatever.
A battle tower... A big black obsidian tower where you have a challenge to fight on every floor. I just copy pasted the design from one floor so it was 20 floors tall. These all worked with dispensers that triggered when you walked into the room. At the top there were a variety of challenge rooms (a forest maze, indiana jones style pressure plate arrow traps) and at the very top was a huge open area where you fought 3 ghasts at the same time. Oh also, every few floors, there was a chest where you could upgrade your equipment, so you start out with leather stuff and end up with enchanted diamond everything.
This was cool... There was a huge mountain and a ravine next to it. I made a sort of miners shack over the ravine, with a mine cart track leading to the bottom. At the bottom, there was a lever which ran to some TNT - the miners must have been digging! Triggering the lever blew up a wall and let you explore inside the mountain a bit, which was filled with jewels. But triggering the lever ALSO sent up a redstone signal all the way to the top of the mountain, where I had pistons holding back lava. Basically pulling the lever turned the mountain into a volcano.
A fishing hole. This was actually my favorite to plan out and make, but the kids werent as interested. I figured they might because at least one of them enjoys real fishing. Basically it was kind of laidout like a golf course but using the fishing rod. And the "holes" were all pressure plates in a pond. Around the pond there were a bunch of red wool blocks set into the ground. You are supposed to stand on the wool block and throw your fishing line towards a pressure plate in the pond. Depending on where you were standing it could be very tricky to get the right shot, sometimes you have to fire through holes in leaves and stuff. If you landed on the pressure plate, the red wool block would retract with a piston, and you would fall down into a little pit with some treasure. Then you could flip a lever to make a ladder so you could get back out.
A windmill near my house... There was nothing much special about this except that it was a replica of a real world thing near our house. I also put it full of secret passages and hidden goodies to make it fun to explore.