How is her short term memory? I.e. would she later remember if you said 'I can't take you today/on Wedsney etc., but maybe on Saturday/next Tuesday?" And repeat (with variations) as needed.
Maybe you've tried this and it doesn't work, if so my apologies. However it was how I handled my mother often wanting to go and visit her (long-dead) parents.
In any case, you absolutely must not beat yourself up, or feel guilty or terrible.
Sometimes (or I may say often!) there are situations with dementia where there simply are no easy answers at all, and whatever you do will end in the sort of outing-debacle you describe, or else the person ranting at you for not doing what they want.