If you are a frequent tennis player then there are a few items that you should invest in.
1) a vibe dampener- it reduces pressure on your elbow (reducing the risk of tennis elbow) and makes strings last longer.
2) an overgrip- the added grip from these overgrips is phenomenal, and greatly increases comfort and accuracy.
3) A net measurer- there is nothing worse than playing with a net that is not set at the correct height.
4) A spare racket- you never know when your strings are going to give up the ghost. Make sure you find a good local re-stringer too!