State the limit concretely
Choose clear words such as: We cannot host this week, or I cannot take on that task right now. You do not need a long defence. Offer another time or kind of help only if it genuinely works for you.
Review the plan again
You do not need to fill the space with another task after saying no. Rest and unplanned time can be part of family life. Discuss preferences openly with people who share care responsibilities.
Your next little step
Choose one idea that suits your family and try it at your own pace. Every family has different needs.