Sorry - I probably should have done a search!
I just feel like he "gets it" - I could say, "Yes, I have the evening free. I would rather spend it alone than with you - not because I don't care about you, but because I'll be a better girlfriend (and happier overall) if I can have some time to myself." He would probably reply, "GREAT! I have some stuff I want to do alone too. How about we meet up tomorrow instead?"
Then we would both go do our thing and it would be great, and I wouldn't have to worry about being guilted for needing some down time.