I'm under 30 but for most intents and purposes I am "retired", my situation most like is very different from yours, but this is how I deal with most of my free time (which is a lot)
- Gaming of course, sometimes I'll go days without playing and then sometimes I'll play 5-6 hours in a day, it comes and goes, my point is, don't force yourself to game. Only do it when you actually want to.
-Excercise, excercise, excercise, go to the gym, go walking/running, play a sport, maybe join a team, there's plenty of sport alternatives for retired people, depending on your health of course. Excercise is one of the main things that keep those feelings of uselesness and depression away, the better shape is just a bonus on top.
-If you want to study, there's plenty of free courses online for basically any topic, you just try different things until you find something you actually like, youtube is also great for learning DIY stuff.
-Be Social, reach out to people and make an effort to share activities with others, I'm not an introvert but for me its really easy to get lost in my own space for days and be completely isolated, not because I don't want to be around people, I just feel like I can't be arsed. You have to go out of your way to make stuff happen now that you don't have a job that forces you to have social interaction. Humans are social creatures.
-If you have a partner, talk to them, get involved with their hobbies and activities a bit more, maybe you'll find a shared passion and that will make the mundane a ton more fun. My wife picked up videogames cause of me and we have a ton of fun now, currently playing Far Cry 5 and having a blast. In return I've picked up cooking and biking.
Time is a precious resource, now that you have more of it, time to make the best of it. Don't feel bad because you're not "contributing to society" anymore, you've paid your dues, time is your reward, enjoy it.
Also, if you're an animal person don't be afraid to get a pet, having someone/something to care for will take a lot of your time and make you feel responsible towards something, I love my dog and caring for her gives me a ton of satisfaction.