The West Reservoir Centre off Green Lanes (next to the Castle Climbing Centre)
The St John's Deaf Community Centre, also on Green Lanes--not sure for how long they'd allow you to be in there, but would be fairly cheap (or at least it was when I enquired some years ago).
Hackney Downs Studios, specifically the Russet Café
Some more I forgot earlier:
The West Reservoir Centre off Green Lanes (next to the Castle Climbing Centre)
The St John's Deaf Community Centre, also on Green Lanes--not sure for how long they'd allow you to be in there, but would be fairly cheap (or at least it was when I enquired some years ago).
Hackney Downs Studios, specifically the Russet Café