Simple Night Out
If you’re the kind of people who prefer simple things (kind of me, sort of–although I do like a good going out to eat, just FYI), you can just go out for a nice evening walk or go on a picnic and catch some Pokémon on your way there and during.
You can get all romantic and maybe even watch the sunset or hang out by a Pokéstop and lay down a lure to attract all kinds of different Pokémon. (Warning: lures also tend to attract people, so if you want a kind of quite date-night then I don’t suggest laying a lure down).