BBQ would be great!
It's a PITA to get to though... and are there any public toilets nearby (seems to be a necessity, if large quantities of booze are involved).
It's close enough to walk from Mile-End or Bow Church and there are buses from Mile-End that run right through the park! I believe there are public toilets beside the cafe in the smaller (westerly) section of the park. There may also be toilets in the larger section at the changing rooms for the sports pitches.