@SophieAdams ... the asian leafy vegetables are great for quick cropping, like tatsoi, mibuna, bok choy, mizuna, choy sum, garland chrysanthemum etc etc.Classic asian cabbage like Michihili is high yield in a short time - I think it's called napa cabbage in the Americas.
Also Daikon radish - these all give a big yield and high nutrition in a short time compared to resources used.
I'd recommend at least three asparagus plants - cropping for 20 years is a great return for less work. mulch and feed, you're good to go, and you get the ferns later to provide shade for some other crops in summer, or a pretty backdrop - depends where you are. They also give you some free mulch when they die back!
Honestly, so much depends on your length of growing season, average temperature, frost situation etc.
The others have pretty much said everything, but if you can trellis and grow climbers like beans, peas, tomatoes, squash and marrows, you'll get a great return for space used.
