An ACD mix would probably be fantastic at agility- a good way to exercise both her mind AND her body.
There's a good chance she'd really enjoy playing a bit of frisbee as well. While I don't know a ton about it, my best suggestion for getting her started would be to start really small. Try sliding the frisbee across a floor, or rolling it on it's edge across the ground. The trick is to get her interested in seeing the disc as an object worthy of chasing/hunting. If she seems to be enjoying that, and retrieving the disc relatively reliably, you can graduate to short tosses. Don't worry if she's not catching it from the very beginning. It's defintely a learning process, and I think you'll be pretty amazed to watch that ACD brain start ticking- it's always a treat when you can actually see a dog *judge* a frisbee flight pattern.
Work on doing short tosses *to* her as well. Don't throw it straight towards her face, give it a little arc up in the air, and see if she tries to go for it. It may take awhile for her to catch on, but I bet she will.
I know some people have managed to peak their dog's interest in a frisbee by feeding them out of it, LOL. A weird tactic, but it worked for a few acquaintances.
Good luck with her, and have fun!