You could play the guitar, but everyone plays the guitar. You could play the cello, but cellos are massive and aren't that great around a campfire. Now the ukulele, now there is something that is small and growing in popularity. Grab a Kohala Soprano ukulele KU/HU212 with songbook and chord chart for $49. 

Specifications:

• Built better with nato top, back and sides and sounds better than your cheapies.
• Vintage geared-style tuners for stable tuning.
• Includes a song book with half a dozen songs, tips on how to tune your ukulele and strumming to get beginners and hobbyists playing right away.
• Good dark brown finish - It’s not a toy, this one’s a real instrument.