You could put out pansies, which like it cool, but not the more traditional summer annual flowers, which cannot take a frost. If you can work the ground, it is safe to sow vegetables like spinach, beets and radishes. For more ideas, check out the cool season vegetables on this list from Penn State's Cooperative Extension service:

http://pubs.cas.psu.edu/FreePubs/pdfs/xj0026...