It's good exercise and I want something that can be done throughout life. I also want whatever I provide to be useful to the community outside of organized competition. Finally, if it's something I'm donating millions to, I want it to provide something that isn't necessarily accessible otherwise.

Tennis courts and baseball fields don't have much secondary purpose beyond whatever sport they are for. Same with outdoor basketball, though indoor has more use. Football, soccer, lacrosse basically are big open fields which are always useful....but also just land, so I'll let the city deal with that since I've got absurd amounts of money in this case

Putting a bunch of youth pools and swim teams around the city/state provides something that isn't otherwise available, can be used by individuals (don't need to get a team together to use it) and can be used by many in a way that supports lifetime fitness