BTW OP, please try not to be all catsbum about any money spent by them, my parents are like that, they like cheap holidays, saving, being frugal, and while that has a place, they also would consider spending that amount on 'just a holiday' immoral.
At my wedding, everything was 'waste of money. My dad rolled his eyes at everything I told them about so I stopped involving them/discussing anything with them. Effectively they were guests rather than hosts, they didn't know what was going on when, because I just couldn't be bothered with their complaints at how we spent our own money. It takes a lot of energy to manage a judgemental scrooge, most people with parents like that either don't do what they can afford and want to do in order to get their approval, or else they just stop involving said frugal person.
You don't have to say anything, but just by giving looks or making comments about going on cheap walking holidays you are passing judgement. You are setting your DIL up for deciding that you don't get to know details or being involved in their lives because you'll judge. Do you want your DIL to spend the rest of her life not discussing things with you because she can't be bothered with working out how to manage your reactions to them spending any money?
I don't feel I can have sensible conversations with my parents about anything that involves spending money because they judge like mad. For example, they offered to buy a pram for us when I had DS and I refused because I just couldn't deal with having to have the cheapest possible regardless of how suitable it would be for us as a family because dad hated 'waste' - even when we could afford the one we wanted without their help. they missed out on shopping with me when I know Mum wanted to come looking at prams and baby things because I knew he'd walk round all catsbum at the prices of them and then nag me to buy the cheapest even if I was paying myself. It's just so draining dealing with parents who judge your spending habits even when they aren't being expected to fund it.
Bearing in mind most of the time MIL and FIL see DS it's because I've arranged it, not DH, you don't want to piss off your future DIL over something that doesn't actually effect you in the slightest, it's just you seem to see it as a moral issue. beak out, smile on face and say "gosh, that sounds like fun." not "how much is that going to set you back?"