I can't stand getting something like, "in lieu of a gift we have donated $$ in your honor to..". Let people support their own causes and not tell me it is a "gift" for me. But on to your other point, tell your relatives you don't want to exchange meaningless gifts. I agree with you that after a certain age it gets REALLY hard to come up with something ever year. We still exchange with our parents but not with our siblings. We do buy for their kids who are little right now. Speak up. You might be surprised and find out they want off the hook also. If not, at least you will know you tried.
If I want a donation, that would make me much happier than some useless present. I don't mind at all if someone wants to donate to some worthy charity. I think it is great