You could walk around a bit if you like but don't pace or have constant movement - you'll disorient the audience likely. Hand gestures are okay I think as long as they appear realistic and are used sparingly.
Engaging the audience - I think most of it is within the enthusiasm and conviction in your voice. Some of it is to do with eye contact (perhaps focus on the centre but pan occasionally so the sides don't feel totally ignored), and just general confidence in stance and all that. Obviously your posture should be upright but not mechanically so. Careful for crossed legs, hands in pockets, etc. which can make you appear casual. And yeah, just use common sense and don't forget to pace yourself and talk in a somewhat natural way.