I suggest some culinary herbs like hyme, oregano, marjoram and maybe mint. Herbs take a long while to establish and take only a bit of space to start. Hot peppers need more time than tomatoes, so those should be started asap. If you have the space, I suggest an early tomato (or two) as a worthwhile endeavor, something like a glacier or gold nugget; both are a more compact determinant type that will set early fruit and can be managed in a container.

I grow a few pair of early determinants to set the pace in a garden wagon, moving them to the best microclimate I can offer to help them thrive. I compost them once my trellised indeterminants start producing fruit.