I'm only in my second year of veg growing myself and I have four raised beds at the back of my garden (4' 4" by 13' each). I would suggest if you can still find first or second early potatoes in shops then you could get those planted now. You could sow a packet of leek and or onion seeds in deep containers asap ( I use root trainers, others have advised using a cat litter tray as an alternative). If the growing depth available is deep enough the leeks could then be left to get to pencil thickness before planting in the space left by the potatoes when you dig them out.
Other things you can still plant are peas and beans, carrots, beetroot. I'm doing brassicas ( just a few at a time as they will take up a lot of room when maturing), I'm doing celabrese, red cabbage and kale( which I read can be used as salad leaves when baby leaves and can be grown to maturity right through the winter months as apparently it's as hardy as veg gets). In between you could do lettuces, spinach, rocket, radish. My beetroot seeds are interplanted in my brassica bed ( not strictly in accordance to rotation though)
I planted onions, shallots and garlic from sets last autumn so they are quite far on now, something to think about when this autumn comes.
I have heard that celery is quite hard work.
I'm sure more experienced posters will have lots more ideas ( and correct any that I've got wrong).
Good luck and enjoy, I'm really loving my new veg plot