Well, if I hadn't been living with her longer than I wanted to...
I might have held up the butter so she could see it and say "Got it!!", or I might be silly and say, "Well, I've decided that I am going to forget the butter today. I haven't decided yet about forgetting it tomorrow." , or I might tease and pretend to be thinking out loud and say "Hmmm, is this the very first time I've set the table?? No-o-o-o...."

jbug, it's perfectly fine to say that you are needing time alone and so you won't be going on the road trip with them.