Excluding gym instruments, best exercise to strengthen and make them look good and healthy?
Your thighs will benefit most from focusing on two separate muscle groups which will require separate exercises to target. These two groups are going to be your quadriceps ("quads") which are the four muscles on the front side of your thigh, and your hamstrings which are a group of muscles and tendons on the back side of your thigh. As a bonus, the glutes are often involved in many hamstring exercises. In order for your muscles to grow you should work them to (or close to) failure within a set, and 2-3 sets a day 2-3 times a week should be plenty to see results.
Best Exercises for Your Quads
- Pistol Squats: Because this exercise only utilizes one leg at a time, it can give your quads a good challenge and stimulus for growth. Easier variants include the "Bench Bottom Pistol Squat", and the "Levitation Squat".
- Split Squat Hops: This would be the more dynamic option for your quads. It will help to train for explosiveness more than shape.
Best Exercises for Your Hamstrings
- Glute Ham Raise: The only trick to doing this exercise without equipment is finding a way to anchor your ankles into place. Be it a friend, furniture, or whatever happens to be around, this will be your best bodyweight option for training your hamstrings.
- Slick Floor Bridge Curl: This one also comes with a catch, you'll need to find both a slick floor and be wearing socks. Otherwise, it's a very accessible exercise.
- Hip Bucks: These are a final option, though less challenging than the others. It should also be noted that these are also primarily a glute exercise, but the hamstrings are certainly worked here as well. You might try doing a single leg variation to make them to reach failure within a more appropriate range.