French beans! Open pollinated types are true (they don't cross pollinate easily), the beans are yummy, and you can have bonus haricot if they crop too heavily. I have a soft spot for the 'Brittle Wax' Lidl sells - dwarf yellow, very tasty, and less than 50p a packet to get you started. Tall types are heavier-cropping - I like 'Cobra', although I'm also trying 'Cherokee Trail of Tears'; both are black seeded, so if you want white haricot go for 'Blue Lake' or something.