An alternative to singing that a lot of instrumentalists use is story-telling between tunes. It breaks up the monotony and gives the audience members' picking receptors a chance to recharge. Some of the best guitarists I've seen perform don't sing a lick but are excellent and engaging story-tellers. Ed Gerhard, Richard Gilewitz, Richard Leo Johnson and harp-guitarist John Doan come to mind.

Problem is that unless one is just a natural-born story-teller it's difficult to pull off effectively. It needs to seem natural and unrehearsed, a goal which, paradoxically, can only be accomplished by lots of rehearsal. It also needs to be free of excessive rambling, have some connection to the music, and not come off as just a bunch of jokes delivered by a failed stand-up comedian.