I read Peggy Post's column recently, and she wrote that the only obligation for an overnight guest was a gift and thank you note, and that if staying more than one night, then dinner out is nice, but not mandatory. I'll be staying the weekend at my brother's this summer on the way to another destination. Do you agree that a gift and thank you note is sufficient?
For guests staying more than one night, in addition to a small gift and thank you note, they should at least offer to take the hosts out to lunch or dinner. It doesn't need to be any place fancy, but it gives the hosts a break. The hosts are putting you up for the weekend, and when you're gone, they will have to clean up, do dishes, laundry, etc. The hosts may decline the invitation out, but it's always nice to ask.