The Ibanez IBZ15GR is a small rehearsal/practice guitar amp with 15 watts of power and an 8" speaker. It's light, affordable, and almost everthing you need in a portable amp, including tone. 

The IBZ15GR is designed for electric guitar, but you could use an acoustic guitar also. Other features include closed-back cabinet for increased bass response, a separate Distortion channel with Gain and Volume controls, and reverb. There's also a headphone jack for private practice (and to keep others off your case), and an aux input for your iPod, CD player, etc. 

Turn on, plug in, get happy!
Ibanez IBZ15GR Guitar Amplifier With Reverb Features:
15 Watt Guitar Amplifier
8” Speaker
3-Band EQ
Reverb
Headphone Out
Aux input for iPod, CD, etc
Closed back design for increased bass response