Just leave. Setup your Marriott profile to have your final bill emailed to you. Or call Marriott after the fact and they will email or fax your final bill to you.
I haven't stopped by a front desk for checkout in 10 years unless there is an issue. Even with the receipt slipped under the door in the middle of the night, people feel compelled to formally "checkout", I've never understood that.

Being cheap, I'd try to avoid paying for the night with some of the excellent suggestions above. Asking for a later checkout and using the pool/spa facilities are great ideas. Most pools have locker room / shower facilities too.