Just did a my sweet solo version of Voodo child (slight return) at a school concert. It was awesome! A 3 minute long improv session...yummy. But try not to do songs with too much rythm and no vocals! Pick something with an interesting guitar part. You might be able to get away with some satriani, or you could play van halen's eruption. that usually impresses the crap out of people, and its pretty easy to play. Or you could do an all out shredfest, it's up to you.