Along the lines of spring cabbages, I'm about to sow purple sprouting broccoli for next year too.
You can also get potatoes that will still come through for this year - I've got some coming that will go in in June, ready for Christmas eating.
Also fast to crop stuff (especially given that much of my April and May stuff is not very advanced) - carrots, beetroot, peas and beans, lettuce, radishes can be sown from scratch. I've only just planted out tomatoes, sweetcorn, pumpkins, cabbages, brussel sprouts, cauliflower and broccoli too so if you can buy plants of those it's worth a go.