You do not need to stay housebound just because the weather is crummy.
Although it may not be a traditional date, a trip to the grocery store, mall or library could prove surprisingly romantic. These low-cost, casual environments take away some of the pressure of needing to provide a romantic atmosphere, and allow you get to know each other through casual conversation. Purchase ingredients for a dinner that you and your date can cook together, or check out a romantic book of poetry to read to your significant other.
If you decide to stay indoors, figure out an activity to avoid rainy day boredom.
Ask your date to think of a skill she can teach you -- for instance, upgrading a computer, making an exotic cocktail or dish, or saying phrases in a foreign language. You will come away knowing more about each other.
There's nothing more cozy than watching a movie at home or a playing board game with your date.
Try choosing two games -- one you are skilled at playing and one he is more skilled at playing. See if you can beat each other's scores. Head to a local coffee shop, if you want a change of scenery while you play. If games are not your thing, rent a movie -- scary movies are especially suitable for rainy days.
If you do not feel like staying indoors, and the rain is not too strong, take out your rain boots, raincoats and umbrellas and head outdoors, anyway.
Try strolling around a nearby park, neighborhood or downtown area -- the rainy day has the benefit of reducing crowds, so try heading to an outdoor attraction like a zoo or aquarium. If you live near a scenic area, take a drive for a compromise between indoor and outdoor activities.