Medicine ball such as 3kg for an adult and 1-2kg for adolescent is an awesome investment to improve your balance, patience and leg-eye coordination technique. Learn to balance on top of the medicine ball to work on your core, vital to fast turns and more advanced shots in soccer. Learn to keep the medicine ball on your leg and then proceed to real ball -- it is extremely motivating to see how fast you develop your technique!
If you cannot afford medicine ball, put a rock inside a styrofoam or tape and do your ball with some weight. Also experiment with some physics and put the weights on the exterior of the styrofoam and you realise that you are able to get a ball of similar properties as earlier with the weight in the middle but less weight.
My favourite thing while training alone is to use Medicine balls. You can use them for weight training, for technique, for massage, for anything creative -- go to a silent basement and learn to stand on top of medicine ball while getting support from the wall is one of the wow effect: little by little you learn new things and suddenly you realise how awesome you are getting in your technique proceeding from your basement to the fields.
Motivate yourself with awesome things such as
Freestyle Football: a lot of technique training, youtube with a lot of videos such as here
Balls of different weight: developing coordination and balance is easier with heavier balls because mass has its slowness, more mass so more force needed to adjust the ball -- my favourite are Medicine Balls such as 1,2,3,4 kg
Balls of different sizes
The good thing in medicine ball is that they are timeless.
Whatever your age, you can find them. You can find medicine balls of different weights and they don't get old. 1kg ball is awesome after an intensive game to relax your muscles. 1kg ball is equally awesome for a kid to feel his/her muscles better in his/her legs. If a child is not ready for 1kg ball, have an orange in a picnic or play some other round thing on April's fools day with your children.
Improving ligaments and joints early can safely be done in many other ways. The ways contain gymnastic, yoga, skateboarding, surfing, horse riding, dancing as well as balancing with medicine balls. Choose a method you like most and combine freely together as you wish. And remember to drink a lot of water so your joints, ligaments and bones are more resilient and no injuries in trying new things. Have fun.