Raspberries are the easiest to grow I ve found. They don't mind heavy soil either. If you re sticking em in the ground be prepared for canes to be cropping up all over the place.
Thornless blackberries. Also very easy to grow - think the resilience of a conventional blackberry without the thorns.
Rocket, baby leaf salads also easy (and cheaper than buying bags of salad in the shop). They grow quickly and can be cut when young to encourage new growth.
Chard is an excellent veg which is easy to grow, colourful and can be eaten in salads or cooked like spinach. Leaves are best picked young for salad.