When do you want to return to CLE from SEA?
If it's a few days later, than you should have no problems getting a circle trip CLE-EWR-SEA-CLE at a reasonable price. With a Saturday night stay in SEA, I pulled up a boatload of $450 fares on Continental (added benefit: two of the three are non-stop flights).
Without a Saturday stay ... well, there's a few ways to get around that too, but we'll cross that bridge when we come to it.
Mook