In really slow/easy lectures that require attendance for participation points, I'll bring notecards to go through, update my planner, or something of that sort, to do while I'm there. It helps me stay productive and focused on something when the other thing isn't engaging enough to keep me focused the whole time. A lot of these classes just require we answer questions with the i<clicker, every 5-10 minutes, so most people bring something else to do. It's not just an ADHD thing.

In other lectures where there isn't a participation grade or notes to take (like they are already online), but the material requires more effort, I will put the notes on my iPad, bring notecards or the textbook along in case it gets really slow and I feel like I'm not getting anything out of the lecture. Going through the material on your own will help you stay focused on the course content. From my experience, people with ADHD are very good autodidacts.