Tomatoes work beautifully for a May planting. Whereas flower seedlings may be small, tomato plants are SUPPOSED to be small and NOT in flower.
I WS them on March 1, they germinate about April 1, I transplant them into individual 3-4" pots about the last week in April, and they're beautiful stocky seedlings ready for sharing about the second week in May. And already hardened off.
Of course, maybe you weren't considering veggie plants.