Gymnastic rings! You can buy a pair for around $40 and they're definitely the best value item at that price. If you use these, your grip strength will increase tremendously. Plus, you'll get kickass workouts anywhere you go.

http://www.amazon.com/Yes4All-Olympic-Crossfit-Gymnastic-Flexible/dp/B00IGYU6Y2/ref=sr_1_fkmr1_2?s=sports-and-fitness&ie=UTF8&qid=1426906393&sr=1-2-fkmr1&keywords=gymmastics+rings