We would need to know your space limitations, really, as Mum says. This is important.
The fruit, once established with good pruning and feeding can produce lots. I have that much red currant on one established bush that I struggle what to do with it.
Feed established strawberry bed with tomato feed. This is good for a lot of flowering crops.
Read the description carefully when buying seed and plants about the variety, some are prolific(they will be).
Buy reproducing broccoli, there will be second, smaller heads to harvest.
Try Central-Eastern-European and Turkish pepper varieties, they shall be more productive than a Bell pepper kind. Generally try more natural varieties than those produced simply for the shelf-life.
Burpless Tasty Green and Crystal lemon cucumbers are hard to beat in performance in my book. Both do well outdoors, the latter even in the worst of summers. The first you only need one plant of, yet again, upwards. You get such in marrows, too.
Try dual purpose plants: celeriac will give you a bulb as well as leaves, the same with Hamburg parsley.
But then, good husbandry is key. A good rotation, feeding and watering routine, that the plants need is crucial.