I grew cape gooseberry or physalis and cucamelon this year both grew very well I'm in Sout east uk
Physalis has a citrusy flavour - delicious.
What about mini pumpkins? Sarah Raven sell some (I'm sure others do too). Not overly unusual but a definate cute factor.
grew cucamelons last year, pretty and also pretty useless, cape gooseberries we found were a bit too tart for most people, maybe we picked them to early?
we're already growing courgettes, jack o lantern pumpkins as well as summer squash and our site isn't that massive. So I think more pumpkins are a no go.
We grew Okra in our poly tunnel and it has very attractive flowers and taste much better than any shop brought.