Sons of the Desert is very funny...less slapstick, but great comic dialogue and situations.
There is also one called Battle of the Century, which has the film world's largest custard pie fight. It starts off with just Stan and another man quite by accident. Ollie gets involved and then it escalates into over a hundred or so extras getting involved. Imagine the canteen food fight scene in Blazing Saddles multiplied by a hundred and lasting for about ten minutes! Loved it as a kid, so I showed it to my twins and they couldn't stop themselves...I'm sure one of them had an 'accident' in his pants by the end!
Now THAT'S what comedies should make you do!