For just 20-30 bucks a month, just 20-30 bucks a month, you can have access to tens of thousands of dollars of gym equipment, a basketball court, and a swimming pool. Swimming is the easiest way to develop strength and flexibility in your entire body, and you're doing cardio and developing a strong diaphragm at the same time. You can just tread water all day, do over exaggerated running motions, shadow box underwater, all kinds of crap, and develop loads of strength and endurance.
But if you just want stuff at home, here's what I recommend getting in the future. I know you want to develop strength without any apparatus, but trust me you will see better and faster results by incorporating weights into your routine in the future. Get that gym membership, get some friends to join, join the community, I highly recommend it.
Olympic rings to hang up somewhere in your garage or backyard or wherever is convenient.
Any pullup bar that attaches to the top of the door frame.
A set of adjustable dumbbells. Not the expensive quick-changing ones, the ones with plates you put on. I got 3 pairs. Some shitty walmart 20 lb pair made of plastic/sand, and two pairs of steel handle and iron plate dumbbells. Squats, deadlifts, shoulder press, weighted situps and body twists and oblique lifts, curls obviously, other shoulder and back workouts, and bam I get a pretty good full body workout from just the dumbbells.
A bench for the dumbbells too.
Ok but besides that, at the very least, get a pull-up bar. It counts as body weight stuff. You won't develop jack shit in your upper body if you don't do front/back close and wide grip pull-ups. Find something to do dips on. Do tons of push-ups. Do a set of push-ups every other minute, literally any free second you have, do push-ups. Same with squats and deadlifts. If you don't wanna buy dumbbells, use your books. Squeeze two textbooks with all of your fingers. Squeeze them with just the finger tips and not your palm if you can. If you can't lift the books with that grip, work on it. Constantly squeeze books or some other solid and heavy thing as you mindlessly do dozens/hundreds of squats. You will develop your forearms, shoulders, back, and legs so well by constantly carrying something, and just constantly doing squats and deadlifts, and again remember to keep doing the push-ups, and pull ups. Even if you can't lift yourself all the way up after a while, just hang there and keep trying to lift yourself anyway. You will develop insane grip strength, and really hard muscles all over your upper body. Your fibers are still constantly tearing and getting all kinds of crazy stimulation, and not to mention all the neuro connections you are building, by just holding that flex and hanging there.
Do hand stands against a wall, and once in a while lightly push your feet off the wall and try to balance. Or start by sitting down and lifting yourself with your fists into the ground, and then slowly lift your legs up from behind you and try to do a full hand stand. The slower you go up and down, the more thoroughly you'll develop each muscle fiber in your back and arms. You also develop your abs when you just stay in the seated position with your legs in front of you. Try to twist left and right to develop the rest of your core as well.
Hand stands are great for developing arms and your chest and back. You will notice your whole upper body getting bigger from incorporating hand stands, with or without using the wall to keep you up. Try to do hand stand push-ups against the wall as well. Hold yourself in the lowest and highest position, and each position in between if you can stand the pain.
Try to maintain push-up, dip, hand stand, and any other position. Get your body used to being under tension, just like how we're basically flexing our backs and legs every day from standing up a lot.
And also work on your posture. Sit up properly whenever you are sitting. Don't slouch or even lean against the chair rest, and that too will develop a strong back and nice posture.