With about 45 days remaining until the first expected frost, look for greens crops that will mature quickly. Top choices include the turnip variety 'Seven Top,' and various mustards. Mizuna, tatsoi and bak choy. These Asian mustards will all mature before it gets too cold, as will both curly and red European mustard varieties. Also in the mustard family, radishes will have plenty of time to mature before frost. You can get in 2 or 3 sowings, a few days apart, between now and the end of the month. Another good choice is arugula. Plant small amounts every few days for a succession of crops in October.
All these greens are good in salads when they are small, and can be used as cooked greens when they get larger.
Garden centers also have lettuce plants in cell trays. If you don't already have some lettuce seeds germinating, the commercial plants will give you a quick crop or two before frost reduces the quality.
While you are at the garden center, why not purchase a row cover tunnel for your garden? Using one of these tunnels will protect late crops from frost, and extend the growing season by a week or two at least. Given the cost of fresh salad greens, the small investment in a row cover is paid back the first season in our garden.