Well, if she doesn't want a spa weekend, (does she like that sort of thing? Some love it while others think it self-indulgent), is there any other sort of activity that she would love?
In your dating days, what did you do together? What about cultural stuff - theatres, cinemas, concerts?
Does the day - or night - out have to include yourself, or would she prefer a complete break - from everyone - for a few days?
Between postings abroad, I was the sole (and main) carer for my mother - who has dementia - for around a year in 2012, and I can state, with utter conviction, that there is no more draining, soul-destroying, exhausting, relentless, and stressful job on the planet. To be honest, working in war zones (and I have done that, too) is nowhere nearly as stressful as caring for someone with dementia, or autism. This is because you know it will never get better, so the clutched straw of faint hope - and the optimism it allows - is entirely absent.
Whatever you do, it must be something that is solely focussed on her likes, choices, preferences, activities, something that allows her to be herself, (even if briefly), relax as herself, to recognise her own needs without guilt, and something that recognises her as a person entirely independently of her caring role and responsibilities - because that is how carers come to be seen, merely as an adjunct to the person they are caring for.
In her situation, a week-end of respite care would be very valuable, because that job as a carer is a non-stop, relentless source of endless responsibility.
.
