You should study accounting. I have a friend who is an accountant. He works 3 months each year during tax season, then spends the rest of the year rock climbing.

Nursing or PA school can also be good. Once you are able to travel nurse, you can pick up 3 months stints to make money, then take off as long as you want to do whatever else. I have another friend who does this.

Careers where you can make lots of money are also a good option, like tech or finance. If you can manage to get a very high paying job early in your career, you can leverage it to make lots of money at smaller firms later while negotiating for large amounts of time off.

Beyond this, consider going into some kind of trade. I have friends who work as roofers, sparkies, carpenters, GCs, and rope access techs who can all pick up work basically whenever they want.

I would recommend against getting a degree in biology, environmental science, geology, outdoor rec, or any related field. Friends who took this path generally failed to find jobs in their fields, even after getting advanced degrees. The advanced degrees tended to be extremely stressful, expensive, and time consuming to get. If you want to work in national parks/forests, it is not hard to get seasonal jobs as a bartender or tour guide. Working for the park itself often does require a degree, which tends to be a bad deal - bad pay, only seasonal, hard on the body, very competitive.

If you are really dedicated to getting paid to hike, you can pursue a career as a hiking/backpacking guide. Be aware that you will be very, very poor. These jobs tend to be very location-specific, so knowing all sorts of things about the ecology, geology, and history of an area will get you an edge. But the biggest skill here is people skills - the ability to meet a stranger and like them, and get them to like you, and then keep the good times rolling for several days in the woods where you all have nothing to do but talk.