If the baby's taking her morning nap and you have an hour...
Pour a couple glasses of wine and look through your honeymoon photos or wedding album together.
Play a board game or a few hands of poker. Loser has to do the winner's bidding in bed later that night (even if you don't get around to the actual sex, just talking about it can be fun).
Take a 10-minute shower a deux, spend the next 40 minutes having sex, and then cool down for 10 minutes before baby wakes up.
If your teenage neighbor is sitting for the afternoon...
Reminisce about the most fun date you two went on before you were married (baking cookies together? going to a high-school football game?), then try to re-create it, suggests Cyndi Haynes, author of 1000 Best Dating Secrets (Sourcebooks).
Have brunch at a fancy hotel, and pretend you had an illicit rendezvous there the night before.
Go to a matinee of an unpopular movie and make out in the back row.
If Grandma and Grandpa have taken Junior for the whole day...
Sign up for a class in something you both enjoy, like golf or cooking. "You'll have something to talk about for weeks to come," says Haynes.
Look in the local paper for a dog show, country fair, or music festival that is unlike anything you have ever done before. You'll either have a great time or at least bond over making fun of it.
Check in to a hotel for a few hours, even if you can't stay overnight. Order room service and stay in bed naked all day.