I took on an allotment in october. So far I've planted:
fruit: 2 apple trees; 4 gooseberry bushes; 4 blueberry bushes; 2 redcurrants; 2 blackcurrants; 2 rasberries; ~12 strawberries; 4 rhubarb crowns.
veg: 4 rows peas; 2.5 rows broad beans; 3 rows onions; 2 rows garlic.
I'm planning to grow carrots, parsnips, beetroot, leeks, more onions, potatoes, runner beans, french beans, sweetcorn, squashes, courgettes, lettuce, tomatoes, brocolli, cabbage, kale, cauli & asparagus.
A whole allotment is huge, so I'm basically going to try some of everything & see what works best, but the book I was given for xmas (the half hour allotment) recommends growing things that either save lots of money or taste much better than shop bought or both. On those grounds it doesn't recommend onions or potatoes (except new potatoes) but fruit is good.
I've grown butternut squash, they're quite easy, but a bit marginal in UK climate (need the whole of a fairly warm summer to do well) so don't always get very big (mine have been mostly large avocado sized!). Other types of squash & courgettes tend to do better.