My typical response to this (if it's not my husband) is, "I'd be happy to help you learn how to do it." I don't need another project to bog me down, but I do enjoy showing other people how to do things. I find this helps me not feel guilty too, because I feel I've offered a fair compromise.
And the few times they've taken me up on the offer, we've had fun, and they've come out with a better understanding of how much time and effort it takes afterwards.