Absolutely nothing, my two girls refuse to budge and, to be honest, could do with not spending anything that I don't have to. Shame as I can see many doing the same due to a combination of weather and finances, and it only increases the economic recovery time.
Where abouts are you and what age range are the kids? We were wondering about going into town and doing the museum today after it was done up (and apparently heaving on opening) but believe it's closed for the bank holiday and there's probably a train strike on I imagine. Best bet may be a movie and out to the park if there is a break in the rain.