L.D.
First, ask him.
However, I usually steer it toward outdoor sports. It's supposed to snow an inch on Friday--maybe just enough of a base for some speed-sledding!
Anything to get these kids outdoors is spot-on. They can play laser tag, go bowling, etc any day of the year. Not that it's not fun, but he'll only be 11 once!
My son is turning 8 on Sat. He's having some kids sleep-over, but we're having an old-fashioned 2-hour party at the house for several other kids, complete with games (some indoors, some outdoors).
We also have an annual treasure/medallion hunt in the yard for prizes (it IS Winter Carnival time in St. Paul, after all!)