LoL. I was in the same situation. They are just stubborn as **** that it aggravates me. No no no my job is laying people off. No no no 4 days is too short. No no Im afraid of sharks.
Old people are stubborn, and in my experience if you put them somewhere they don't really want to be, they're more likely to complain than have a good time in spite of themselves. There's a good chance you'll end up just getting frustrated when your mother-in-law comes back with poor reviews about the food or the resort staff or something seemingly trivial, as opposed to talking about what a great time she had. And besides, when you really think about it, going to Cozumel barely qualifies as visiting Mexico anyway.
You're projecting your preferences onto them and invalidating the mother in law's personal experiences and priorities. You need to simplify the gift to conform to their desires and redirect your energy on some kind of presentation or written testimonial that focuses on and enumerates all the different ways they helped you literally and figuratively.
Hawaii? Seems like that would fit everyone's desires. You get to send em to a tropical paradise. They get to stay in the United States.
Do you believe that you have effectively communicated why you want to give them this gift and what it means to you? If yes, then give them some options and let them choose. I understand that you want them to go out of their comfort zone and for you to get the best bang for your buck, but the truth is that they should be able to decide what they want out of their vacation. As for feeling like you father-in-law is not getting to voice his opinion, have you spoken to him one-on-one? He may truly be OK with any option.
What's the point of "repaying them" by making them go do something they are saying point blank that they don't want to do? If you're doing it for them, do what they want to do, then go do what you want without them afterwards or later. If the mom/wife doesn't want to do it, and the dad/husband is indifferent, he wants to do what she wants whether he wants to or not. Might they be forced to go and end up having fun? It's possible, but not probable, and even then they'd be hard pressed to admit it, and probably fight it the entire way. What would I do, depends if you really want them to go with you. If you do, ask them where they want to go and take them there and do what they want. You've made your suggestions and they weren't interested. If you don't want them to go, or don't truly care, tell them where you're going, invite them and say you'll pay their way, and if they don't go they don't go. It's the thought that counts... question is, are you thinking of inviting them along with doing what YOU want, or do you want to take them on a trip they want? You can't force it to be both if it isn't. Repaying them by "wanting to force them out of their comfort zone" sounds ridiculous. That's two different things.
That was the first thing that popped into my head. Visit Honolulu for a day or two. They can see Diamond Head and the Arizona at Pearl Harbor (if they're really old, they'll like that idea. Ask them if they've seen From Here to Eternity, it'll help), then fly off to one of the other islands, like Maui or Kauai, and stay at a resort on a beautiful beach for a few days. They'll love it. Hell, sounds great to me!
Echoing what others have said. Hawaii sounds like the best compromise if it's within your budget. That said, if this is really about treating them, just go where she wants to go, and not where she doesn't.
You got married when you were a teen? Anyways, I was thinking Hawaii as well or give them money and let them buy or do what they really want to do. If Hawaii and money are a no ... Suck it up and go where they want to go. Perhaps Clearwater is out of your "comfort zone".... Take them there and YOU make it a memorable experience.
You keep using the words "I want". Shouldn't this be about what they want? You're making this about the destination, and I thought from your original post it was about repaying them and showing gratitude. To make it worse, you bring up going to Cozumel, and if they don't want to go you'll leave them at home and you and your wife will go (again)? Sounds like this trip is about pleasing you and your wants. The trip shouldn't be so much about the sites that they'd see, but rather who they are with. That they'll know they have a grateful son in law who truly appreciated their kind gesture. You're losing focus here as the trip won't be the lasting memory, but your kind gesture and spending time with them will be a memory they'll never forget.
Old people are stubborn. Even if you convince her to go where you want her to go, she will make your life miserable on the trip. If your objective is to pay them back and make them happy, why even take her where she does not want to go and will be unhappy??
I completely see what you're saying that you want to make this trip memorable for them and you don't see that FL would be the best choice. However, since this is a trip for them, i think you'll have to compromise in the end. Hawaii is a good choice. Though Puerto Rico would be a next easiest sell IMO. It's a nice blend of domestic and foreign travel. Almost everyone speaks English in the touristy part (old San Juan especially). It's a US territory so you don't have to get a passport. Water and food are safe. The bio luminescent bay is among the top 3 experience I've ever had in my life. It's magical to see water sparkle and light up as you touch the. I would strongly recommend taking the bay with electric boats versus kayaking. I used this company and absolutely recommend them http://bioislandpr.com/home.html. I had a great meal here http://www.yelp.com/biz/josé-enrique-san-juan-2 and would recommend them as well.
Put me in the "do what they want" camp. Typically, when one is able to come up with more than one excuse or reason not to want to do something, they really don't want to do it. The trip's going to be more memorable if you have fun the entire time, regardless of location. I mean, my wife, daughter and I had a kickass time in Dallas of all ****ing places. "Turns out not where, but who you're with that really matters" True story.