Congrats and good luck on your first day of volunteering tomorrow! 🙂
To make a good impression I would recommend being friendly, smiling, and initiating a hand shake. You should also try your best to get to know the therapist(s) that you are observing. Also, make sure that you NEVER look bored! It most likely will give the PT a poor impression of you.
Also, you should be ready for anything. Some sites only allow a volunteer to watch the therapists work, and ask an occasional question, so there can be a lot of standing/sitting. In contrast, in a few of the settings I volunteered at the therapist included me into their patients' therapies. I was able to administer various modalities, interact with patients, feel different structures/ abnormalities, and try my hand at a few diagnostic tests. It all just depends on the regulations at that site.
Finally, as for the number of questions to ask, it is really up to your own judgment. Some therapists welcome a lot of questions, while others can appear a bit put off by it. Simply gauge the therapists reactions to your first few questions. Also, it sometimes is best to ask questions shortly after the patient leaves, or if there is a gap in the treatment time when they are receiving e-stim, or ultra sound. I have also found it much easier to ask therapists general questions (why PT, where they went to school, general Qs about profession) during their lunch break. Good luck, and have fun tomorrow!