Sow a few seeds in a different short row about a week apart; how far apart each row should be depends on the vegetable, ei: lettuce plants can get very large and will need more room so as not to overshadow their neighbors, whereas radishes don't get vey large.

Yes, different rows for all the above. As for the lettuce, I planted a row in full sun back in mid-March (3 varieties). 3 weeks later I planted a row on the shady side (N) of my cucumber trellis . I did this because as lettuce growers know it can get bitter when hot weather sets in (earlier every year it seems). The cukes will shade the later lettuce and hopefully extend the sweet harvest season, we'll see.

Radishes every 2 weeks works here ok before the heat sets in. Depending on your location radishes, lettuces, carrots, and "greens" planted again in the fall do well also. My preference is greens and carrots planted in the fall behind other veggies that have panned out, like melons, bush beans, early cukes, and corn. They just taste sweeter when fall planted (around Aug 25th here). Happy harvest!

I routinely sow my winter crops - beets, turnips, onions, TWO feet apart. That leaves room to slide in the summer crops in between them as they mature. Those winter crops still have large leaves, so I have to prune a bit to allow sunlight to get in, but as those winter crops get harvested, the summer plants (squash, melons, cukes, etc.) get lots more sun. Pretty soon, they take over the bed.