The Ibanez IBZ10G is a small practice amp for guitar. It features 10 watts of power, 6.5" speaker and 2 bands of equalization.
The IBZ10G is designed for electric guitar, but you could use an acoustic guitar also. Other features include closed-back cabinet for increased bass response, a distortion on/off switch and volume control. 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.
It's light, affordable, and almost everything you need in a portable amp, including tone.