A mountain bike. Every measure of my health has improved and I've never traveling all over to hit bike parks and trails. I've spent more time outdoors this summer than I have the last 5 years.
I live in an urban environment, so I don't want to store a bike (or worry about getting it stolen). It's 1) vastly increased my footprint and appetite for checking out new areas and 2) helped me get some more exercise into my days.
Expect to spend about three grand for a good full squish mountain bike. I have a Rocky Mountain and I would highly recommend the brand. They are one of the best values in terms of features, weight, and quality. There are even some really good direct-to-consumer brands now like YT. For mountain biking you definitely want disc brakes, a dropper post, and around 120mm-130mm of front travel with a total weight less then 28lbs. Global Mountain biking network on YouTube is a great resource as well as local riding clubs and bike shops. Good luck!
The major choice to make, irrespective of brand, is what type of bike — which is dictated by the terrain you have available and would like to ride. There are subcategories, but a useful rough split would be: a cross-country (XC) bike for good pedaling performance both uphill and on the flat, but not so capable going downhill and in the rough stuff; an enduro if you want to ride rougher downhill terrain (roots, rock gardens, etc.), but still have a bike that pedals well enough up technical climbs and fire roads; and a downhill bike if you want to ride the roughest, most technical terrain, with the steepest descents, and you have access to lift-served mountains (not recommended to start with ;).
FWIW, I’ve had a such a great time riding the Canyon Torque this summer. It’s a long-travel enduro, really capable on the descents, and perfectly fine to get back to the top.
As for motivation, I would echo the sibling comment. Time in nature, good exercise, a ton of techniques to learn and master, a really sociable scene, such a rush riding tough and steep stuff. You won’t regret it.
[0]: https://youtu.be/EX1Kb6TEyn8 [1]: https://youtu.be/Y_xiq2mczX4A
Giant is a good value brand. They make a lot of components in house, so you get cheaper, "good enough" handlebars etc.
In that brand, I'd recommend a used trance 29 or a new trance x 29, depending on your budget. I'm happy to talk about bikes for days!
Trek, Norco, Rocky mountain are also good value. Just about any modern trail bike is excellent.