I have recently said something similar on another thread, but when my mother went through a phase of wanting to see her long dead parents (I didn't want to say they were dead, since she'd have been very upset and would have forgotten whatever I said so quickly anyway) I used to say I couldn't take her today, since the roads were very busy/icy/closed because of a bad accident/my car was being serviced, etc., '...but maybe we could go tomorrow?'
This always kept her reasonably happy for the moment, and because her short term memory was so bad she never remembered that I'd said much the same before.
By this stage I had become so inventive at the good old 'love lies' that sometimes, for the sake of authenticity, I'd add e.g. 'But I'll give them a ring first - we wouldn't want to go all that way and find them out, would we?'
The things dementia can drive you to! But as long as they kept her reasonably contented, I didn't give a hoot how inventive my fibs became.