LadyBugs is one I can think of a guy playing a girl and Just One of the Guys is one where a girl plays a boy at school.
Can you name a sports movie with a guy dressing up as a girl?
"Girl on a Motorcycle". Starring Marianne Faithfull (her real name) and Alain Delon (some French bloke).
Most of the motorcycling scenes were acted by Billy Ivy, an acquantaince (although much older) of mine - he had to wear a blonde wig.
Motocrossed this girl replaces her brother racing motocross and everyone thinks he is riding