I don't see how there can possibly be one "you should do x" answer for this. Too many factors.
I have been giving it a lot of thought as I am soon to be in a similar position, with one DSD who at 19 will be financially independent and with more disposable income than DH and I have ourselves, and with one DSS still a teenager and dependent on us, and one DD still a small child. DH and I have been talking about when the "Family Vacation" comes to an end - at what point do we distinguish between the children (or in a few years, the child) still living at home, and those who are no longer children. In our own situation, DSD can better afford holidays than we can, and has begun to go on holidays with her friends. We both agree completely that it makes no sense whatsoever for us to pay her way any longer, and it is unfair to the other children if they lose out because we are paying for adult DSD. But we haven't quite decided how to deal with it.
I do think it comes down in many ways to the invitation that is extended. My siblings and I have vacationed together, hiring a place together and dividing up not just the expenses but also the cooking, shopping, etc, but then we all have a say in where we go, what activities we do, and the dates are mutually agreed upon. On the other hand, an invitation extended to a prearranged holiday - "we are going to Spain on these dates and staying in this villa and scuba-diving on Tuesday" - would suggest to me that a)the person invited is free to decline if they don't like the sound of it, but the details are set by the ones doing the inviting, and b)the ones inviting are paying the cost of accomodation and travel.
I'm surprised that spending money is open to debate, though! My DSC have had to provide their own spending money on holidays for the last few years. We do 'treat' them a bit more - an ice cream here or there - and pay for activities that we are all doing together - but if they want to go off on their own and do something, want to buy souvenirs, or want to spend on extras, that's their responsibility. I certainly wouldn't give spending money to a young adult on a family holiday.