- Peds walking up to a soda machine and getting a drink, or walking up to a hotdog/taco stand and getting a snack
Can be done, but I need the machines' model name and a fitting animation/scenario from the game.
Dunno about the animations but I'll see if I can track down the models names.
- Peds actually playing tennis at tennis courts, or doing other activities in places reserved for them, like golfing
Way more complex than it looks, tennis and stuff are entire gamemodes reserved for the players. Doubt I'll be able to code this.
Hmm. Maybe it could be simulated in a rudimentary fashion by spawning peds that simply play an animation cycle of playing the sport (idle animation + occasional timed hit?). There's one mission-related golfer in-game, and maybe the animations for tennis could be tracked down as well. It would be way better than the empty fields we have in vanilla.
- Peds actually swimming in their private pools - at the moment they just sunbathe or sit by the poolside. There could even be a cycle of sunbathing, then swimming, then going back to sunbathing.
I'd need coords for these pools. 
I'll track some down.
Another thing that popped in my mind, that might actually work fairly easily based on the internal game logic would be to spawn an occasional fire in a non-city location. Like a wildfire in a forest area or a burning farmhouse. As far as I've observed, every fire prompts a fire truck to spawn and put it out. Like an expansion of the emergency vehicles you already have added.
Having the rare non-player related fire emergency unfold would be a cool little bit of emergent behavior.
A caveat with that one are the fire stats, in vanilla the fire doesn't really spread and dies out pretty fast so the player might not get to see it - this can be changed from the firefx.dat and I have it set to behave like in Far Cry 2 and 3 in my game.