I know a welsh spaniel who is gorgeous, loves agility but is generally very laid back.
But I wouldn't like to be checking her coat for ticks.
I keep thinking jack russell or fox terrier or tenterfield terrier would be the go for little dog that is up for agility, but it is at least as energetic as a cattle dog.
Personally of all the farm dogs, I think the cattle dogs are the most laid back. Ie they're happy to do their own thing if there are no jobs from the boss to do. They're not like a BC that is constantly in your face looking for something to do.