I'm still planting watermelon and melon seeds directly in the ground. At this point, it's too late for 100 day variety of watermelons. But there are earlier maturing watermelons and melons. You didn't say where you are gardening, but there's a chance you'll still be harvesting some before the end of this season.
Tomatoes are typically started much much earlier -- January~March depending on where you live, so you're better off getting a transplant from a garden center or a nursery this year.
Enjoy your garden and good luck