I think you are safe to do that even if it frosts, I would put them out at the beginning of a cool cloudy period so they are not shocked by the extra sun, or shade them otherwise. I wish I had a high tunnel, I am waiting at least another week to avoid frost, which we usually get here in May.
Lilac is my favorite flower, but so early that farmer's markets are not open yet. You can peddle them to flower shops though. Other early season money-makers are pussy willow, crab apple, and quince. Flower shops also want greens so evergreen bushes like euonymus and boxwood are good to have down the road. Sunflowers sell very well at farmer's markets here, as do dahlias and hydrangea. I used to work at a flower shop and farm stand by the way...