I only have a small garden space to grow in, so I want as much as I can get from that small space.
I don't do squashes/pumpkins as they take too much room - although I have grown them in DMILs and in the past on an allotment we had and they have been great! And cheap to grow as the seed is expensive but the results are plentiful.
I do grow courgettes - well worth it if your family likes them.
I grow tomatoes. I have a hanging basket (a large one) that takes 3 bush cherry tomato plants and gives a decent yield, and I grow 2 cordon types (the kind that grow tall) in a large square pot on the patio.
I have also had a chilli plant in a pot which was great and gave quite a lot for such a small plant. But I find I can't get peppers to grow well.
I do salad leaves (mixed salad leaves packs) and radishes in window-box type pots on the window-ledge or a shelf unit on the patio (that is great for lower growing things and herbs in pots as well).
Brocolli or calabrese is quite a good plant for a small space as - while it does take a reasonable amount of room - you get the main head (like a supermarket) but also lots and lots of side shoots coming on after that over a period of time.
I grow broad beans in the beds and follow those with the calabrese or some leeks.
And I grow some French beans in both the window-box pots (dwarf type) and in a large round pot (18" diameter or larger, climbing types) which are both quite productive options.
I use the "almost dead space" against the fence to grow peas - some years are better than others but they can be good. Sugar snaps or mangetout types can give more crops, as you harvest the crop earlier before they mature so the plant will try to keep giving more to get some to maturity and make seed.
You'll also get more if your soil is fertile - so any (non-diseased) plant material that you are pulling up, veg peelings from the kitchen, and even tea bags and coffee grounds after drinking - all are great to add to a compost heap and then add the resulting compost to the soil before or when planting your seeds/plants. And use liquid tomato food regularly during the growing season, especially for plants in pots, to get more crops from them.
Radishes and spring onions and leeks etc can all take up small spaces in between crops, or to make the lines between different things. Beetroot can make a line slightly larger, and you can eat the leaves in salads as well as any roots that grow.