Plant grass seed
(assuming none of us got arrested for publicly celebrating Naked Gardening Day last weekend). This was best done in the fall, but if you still have bare spots, dig in some organic matter, and spread some seeds. Here are a couple of caveats: 1) Do it before the end of the month, because 90-degree days are brutal on those young blades, and 2) If you already used a fertilizer with crabgrass killer in it, or you put down corn gluten as a weed preventive, the seeds won't sprout. Wait until next year to reseed, and just get some green spray paint to fill in the blanks.
Plant squash. These can go in as seeds or transplants, and the prep is the same. They are heavy feeders, so dig a good-size hole, and mix in plenty of compost. Water the hole, then place the single transplant in the hole at the same level it was growing in the pot or pack. Firm more soil around it, and water again. If planting seeds, enrich with compost, then water the hole, and put three seeds in a "hill" or clump, and cover with a half inch of soil. No need for additional watering until the seeds sprout. Decide which one is strongest, and cut off (don't pull out) the other two.
Prune the forsythia, lilacs, or any of the shrubs that have just finished blooming. Now is the time to make those plants the shape you want them, without fear of ruining next year's flowers. Take out the oldest one-third of the stems on the lilacs to encourage new growth from the base. Chop up forsythia anywhere you please - they're almost impossible to kill this time of year.