Sure you can go to a gym and work out. But there are other ways to improve your muscle tone. Here's an easy, cheap way to improve your forearm muscles, wrist and grip as well as to increase flexibility:
Hold a page of newspaper by the corner and stretch out your arm. Then crumple it up into a small ball as fast as possible using only one hand.This simple movement improves grip and strength and can help in a wide variety of sports from tennis to rock- and stair-climbing, not to mention activities such as opening jars. (Not to mention that it's just one more use for a newspaper.)
The Berkeley Wellness Letter advises that you repeat this exercise a few times, several times a week, as part of a regular exercise routine.