What do I tell my mother-in-law when she says she wants to go on a trip?

AL facilities have day trips. Encourage her to go on one if possible. Or take her for a ride to see the leaves in Fall. After an hour she will be tired, so then you take her back. Don't forget to buy some apples.

Meredee, I see that your MIL is in Assisted Living. I think I would not mention the "fun" things you and your husband do without her. It would be natural to want to share your activities with her, but I'd downplay anything she'd be jealous of.

Does the facility offer outings? Many do things like take residents to a sporting event or out to lunch or to a casino. I'd encourage her participation in these kinds of things and get her to talk about them. Then she'd have a fun thing you weren't involved in.

Other than that, I think you are handling this appropriately.

Just keep redirecting (changing the subject) She probably sees herself the way she used to be. All she is thinking about is going. The process is too difficult. You know what your limits are. Stick to them.

You make the same kind of non committal statements that you would use when you run into casual acquaintances who propose getting together... Oh, yes we should think about planning that some day (not)! You might distract her by talking about trips she has taken in the past, who she was with, her favorite places etc. If she has pictures even better!