My first thought is American Pit Bull Terrier. Active but not too much, shorthaired, 35-50lbs. The only downside to a Pittie is the predjudice you'd have to deal with, and the bans in some areas.

Aussie Cattle Dogs are ACTIVE. They are the type which need loads and loads of exercise.

A Samoyed also sounds good for you, if you're definetly willing to put in the grooming time. They cna be awfully stubborn though.