A no that saves you from resenting someone later is the kind thing to say now.

Saying yes to something you cannot really do postpones the cost and makes it worse. The favour gets done badly or late, and underneath it a small grudge grows against the person who asked, who had no idea they were spending anything. A clean no at the start spares them that. Honesty now is the kinder arrangement, even though it does not feel kind at the time.

Where you are leaning toward a reluctant yes this week, picture yourself doing the thing on Saturday morning. If you can already feel the resentment, say no today. Short and warm, and nothing more needed.