It takes roughly 2 hours to drive to Belize City then depending on what transport you use to get to Ambergris, but 15 minutes or so by plane and 1.5 hour by boat.
ATM is a fabulous cave to experience. A trip of a lifetime would be my opinion. A little strenuous but not enough to not do it! Have fun!
Mystic is really great but their tour and shuttle prices are fairly high. (The 160 is one way.) Searching on line, for Tikal and other tours we used Maya Walks. They will pick you up at Mystic. The Mystic staff were fine with this and even helped us in contacts with Maya Walks.
For the shuttle from the airport, we used Discounted Belize Shuttles and Tours. Their one way is 110.
Hi, I am also a vegetarian and it wasn’t a problem. The breakfast is delicious with fruits and pancakes and other pastries. Dinner offered one vegetarian special each night and they had nice sides that you could make a meal out of. There was plenty of vegetables offered on the menus. I think you’ll love this resort if you go. It was such a relaxing oasis!!
Suggest bringing your own cooler and asking for a bucket of ice from the bar or restaurant. Alternatively, order a bucket of beer/water/juice/soda from the bar and pick up or they will deliver to your cabana on your request. This will be billed to your room.
As previously noted, most all of the roads excluding the main highway from the airport are very, very bad. Oddly enough there are a few exceptions around Mystic River but only short stretches. Saying the roads have serious potholes is very true and maybe even an understatement. I suspect the road quality does also vary greatly with the dry vs rainy season. For example, Mystic River guides indicate some of the tourist sites are simply inaccessible during rainy season due to the roads.
As an apparent resident of Colorado, most of the forest roads in Colorado are tons better than the roads in Belize. So be sure to get the extra, super duper insurance package from rental car company if you do go for a vehicle. As a guest of Mystic River, you don’t really need a car - at least this is true if you sign up for their adventure packages. I did and it was well worth it. Being a passenger was exhausting enough. I can’t imagine how tiring driving would be.