There's always something interesting going on at Grandview park. Sometimes, random bands will play, sometimes there will be free food, sometimes there will be a festival, sometimes there will be a free market (which is exactly what it sounds like). Most days, it's an awesome place to just relax, meet people, and enjoy the day.
If you have kids, the playground is awesome. This park really embodies the spirit of East Vancouver. It is diverse, and "anything goes." It's great for people-watching, and it's equally great for relaxing. You can't really get a good feel for East Van without stopping by here!
There tends to be more shade on the side of the park closest to Charles Street.
Mon-Su: 06:00-23:00