I have been to a couple community days in the park and both times someone had already added lures to many of the pokestops - probably because there is medal for players catching pokemon from your lure.
I would just go somewhere that has a lot of gyms/stops clustered closely together if you have something like that. At the park I go to there are spots where you can barely see the ground because there are so many pokemon. Even stops without lures have a lot, so finding pokemon on community day has never been an issue and I can finish the research pretty quickly.
Tips? Sunscreen and water, even if it’s not hot where you are.
The best bet is to go somewhere where other players are likely to be, places with lots of PokéStops, take advantage of all the lures that everyone places to share.
If you're not sure where to go, you should check out community day map Niantic publishes