Boundaries! I answer invites by honestly stating what I'm up for and what I am not. "I could run to the bank and lunch, if we're back by 2:00 -- but I really am not up for all day, or running a bunch of errands. I just don't have it in me - or - I need to do this or that at home...."
So if you are inviting: Hey want to go for a quick lunch? I have 2 hours open on Sat.
Takes some practice with the persistent, and I'm sure my friend feels it is a bit rude of me. But I'm not going to spend what little free time I have running someone else's errands, so I state the boundaries of my willingness up front.