I grew cucamelon. It produced lots of grape-sized cucumbers with attractive stripey skins. They taste like cucumber with a slightly sharp undertaste. It was very easy to grow and wasn't bothered by slugs and snails, which surprised me. I needed the space so I cleared the plants last week. I now have a box full of cucamelons which seem to keep well. I'll definitely grow them next year.
I've bought several veg recommended in James Wong's book, some of which were disappointing. With hindsight, I realise that a lot of them are flavouring rather than veg, and a lot seem to be used for various alcoholic drinks!
Monarda and Agastache grew beautifully but not good value for space in the veg plot.
Asparagus peas are pretty buy not worth growing to eat - too fiddly and low productivity for the space.
Electric daisies are pretty but unpleasant to eat.
I'm still waiting to harvest oca, dahlias and cannas (for their roots), but the dahlias produced some beautiful and unusual flowers. I'll keep these to flower next year but I don't yet know if they're worth eating!
I also trialled lettuce 'Interred' for Which?Garden. It's a dark red, small cos and I loved it. Tastes like a good cos, looks good and will produce new leaves when you cut the head. I had some seedlings left over which hung around in the greenhouse in modules, dried out several times to the point of wilting, yet stiil produced good heads without bolting when I finally planted them out after about 6 weeks. I could hardly have treated them worse and they grew like champions.