If I read him correctly (hehe communication is another obsession of mine), he asked ....
" Is there a better way to practice without having to go to a court by myself?" And "Are there any exericises or something I could do when i can't make it to a court?"
He can't make it to a court, how is he gonna practice?
"Study tennis" is a nice suggestion but is that exercise ?
Solek, push ups would help you tremendously. If you do them correctly your arms will look like Nadal's. However, you still need to address other muscles tennis requires.
While push ups, weight training, conditioning all will help with the strength/enduance/flexibility, etc., they don't in and of themselves improve one's tennis game. Improvement is the ability to hit more effective shots more consistently and defend more effective shots more effectively. This aspect comes not from push ups or pull ups, but from the understanding of what such "effective" shots are and then working to emulate them with feeling, control, and consistency.
But, you're right that the "study" of tennis is not exercise in the physical sense. (I do consider exercising the brain in terms of visually and cognitavely interpreting what skilled tennis shots look like among other things.)
Shadow swinging in front of a mirror, practicing grip exercises that develop a feel for the racquet...These include bounces up and down, alternating bounces, catching the ball on the strings, bouncing balls down on the edge of the racquet, etc. All of these improve the hand/racquet relationship and helps players gain comfort with uncomfortable grips like continental which is typically not a familiar grip and yet, a very important grip for the serve, volley, overhead, slice and two-handed backhand.
Hitting on a wall can be good or bad. Simply hitting balls over and over, while will improve a player's timing and reaction, it can also ingrain bad habits that don't reveal themselves when simply hitting on the wall. Like hitting on a ball machine, if you are working on poor form, (wrist, poor footwork, or improper swing patterns), you will only get good at being bad!
Be very careful in making sure that anything you do is directed to the concept of more progressive play and nothing short of this will create elements that will very likely need to change.