Spring onions, radishes and quite a few lettuces, rocket and other 'leaves'. If you buy plants rather than seed - then tomatoes and courgettes would be cropping then too - as long as you can find some decent plants to put in!

Little white turnips like Tokyo Cross or Long de Croissy will crop in just a few weeks from seed and are sweet and delicious. Also try one of the small kohl rabi varieties. Otherwise, under Hoya105's 'leaves', you can include various Chinese greens like Pak Choi, mizuna and mibuna greens and so on.