Lots of good ideas here already:
Using animals that you haven't used before or very much. I doing that in this Summer/Fall lull time.. For an added challenge, research them to completion (though I understand that may get boring)
Go back to older zoos and fill out some habitats or foliage in zoos. If you're anything like me, you have some zoos you haven't revisited in a year or more.
Challenge yourself by building something we don't have in game, that you would want. (Prior to the Europe DLC I made my own blueprint Bavarian set)
Make a new themed zoo, in a biome or continent or focus of something you wouldn't normally do.
The "Spin The Wheel" suggestion is a good one if you want the planning taken out of your hands. For less of a challenge, it could be done in Sandbox where money is unlimited.
Since I've decided to not create any more new Franchise zoos until the game is completely finished official support-wise, I see a greater focus on making existing zoos better, rather than opening yet another zoo with a handful of animals that I go back to every month or two. Even with that, I still have about 35 franchise zoos, so I'm not hurting for projects. Of course, my play time has been extremely reduced in the last year, but getting two puppies at the same time (same litter) will definitely do that to free time.