I plant my beets, turnips, and onions in rows two feet apart in the fall and, as they mature, I plant melons and cukes in between in March. They are at first kind of dwarfed, but I remove occasional shading leaves (at least from the beets and turnips, the onions don't shade anything), and in a month or so, the beets, turnips, and onions will have been mostly harvested. The melons and cukes get a big head start this way.
If you want to know what grows well in the summer in your locale, just head to your local extension website. More than likely they'll give a planting calendar for oodles of crops.
In Colorado, you likely haven't yet had your last frost so, as of now, you're looking ahead to a full summer growing season (which is not that long, I guess).
