Quick solution that is not super-ugly: Go to Sisler's in Falls Church and get some big rocks. They have lots of options.
Slower and more expensive, but nicer:
Look into some kind of ornamental grass or shrub that you can plant as a border. If your lawn is sunny, I personally would go for Russian sage. Check out garden centers ASAP to see what they have or recommend and when the best time is to plant.
Or a squash/pumpkin patch.
If your chosen plant isn't available now, you can still prepare the area with mulch, posts, and caution tape.