Spa day.
Big fancy dinner out (one of those places with a chef's table, or a wine pairing meal, etc...)
If money is no object, a stay at The Inn at Little Washington.
Surprise party.
Rent a boat for a day on the Potomac or in Annapolis. Rent a captain to drive the boat if you want.
Buy tix to something (Kennedy Center, Cirque du Soleil, etc...) and splurge on the best seats, a nice dinner, and a limo to drive you to everything.
A night out dancing at the best clubs like you're 25 again and a hotel room to crash in after.