No matter what, smile and laugh. Your face is more important than your legs when you're playing.
Check out some live Reel Big Fish videos - they know how to have a good time even though most of them don't get to move around the stage too much.
99% of the time, I agree. However, some genres appear to require you to look hostile/mad/miserable/stoned... So it depends on the band/gig.
But usually, smiling and engaging the viewers is good. I have to force myself to remember to do that (I'm very metal, you see...)
Re-read the OP - Ska band, you gotta be *fun*. Smile, move around, have fun. Ska = Fun, not angst.