HIE is right about Schnitzi.. its a MUST for all visitors. Also be sure to daven in Bobov 15th / 48th at least one shachris (Shabbos is 10x as good, if you can swing it).
On the outside, Prospect Park / Bklyn Botanical Garden is also very nice. If you’re able to, Bike-n-Roll rental on the West Side Hwy. Flat and really pleasant this week