My thumb tends to be more brown than green so take this for what it's worth...

Berries like raspberry, blueberry, blackberry, etc: are easily grown perennials, but obviously are not available over the winter. I have raspberries and they're easy to grow - in fact, containing them is a bigger problem.

Perennial herbs can be grown outside in the garden or in pots so you can maintain them year-round. I have lavender and sage in my garden and they're pretty bullet proof; the sage in particular is a spectactular grower and I prune it almost down to the ground every fall to keep it from taking over.

Given you're in CO any tropical fruit like banana or citrus is probably a no-go, unless you get a dwarf tree in a pot and keep it inside during the colder weather.

I do have an asparagus bed, it took two years to really start producing enough to pick.

I've never tried to do lettuce, kale, spinach, etc. in a pot but it shouldn't be too difficult to maintain a plant or three for year-round greens.