As an autistic person myself, I own many fidget toys and have tried a wide variety of them. Some are definitely better—and quieter—than others. Here are my thoughts on almost every type.
Starting with Pop Its: they seem fun, but they are not my usual way of fidgeting. I tend to rub my hands together or chew on my tongue instead. In lectures, Pop Its can also be quite loud, so I would not highly recommend them in that setting.
Next are the glass sugar squishy fidgets. Because they are quiet, they are ideal for lectures. However, they can sometimes feel strange, and if you are already overstimulated, their sticky texture may make things worse. They are great when you simply need something quiet to fidget with during a lecture, but they are not the best option when you are feeling overstimulated.
Thirdly, I will consider a fidget spinner. Personally, I find them difficult to use while concentrating, and some can be quite loud. This can be especially distracting if you are sitting near the lecturer. That said, I recommend sitting closer to the front, where there are often fewer people around to distract or overwhelm you.
Another option is a stress ball. It is quiet, easy to use while concentrating, and makes a great fidget. As long as you do not roll it around during the lecture, it can be an ideal way to stim when you are feeling overwhelmed.
I tend to use a scrunchie, which works well as a fidget in almost any situation. It is discreet and can be worn by anyone. Choose one made from a material you enjoy touching, so that you have a comforting fidget readily available if you begin to feel overwhelmed during a lecture. Keeping it on your wrist also makes it easy to reach for whenever you need a little grounding.
Lastly, there is the chewy fidget. I recently bought these (link below), and they have quickly become one of my favourites. You can wear them as a necklace and fidget with them as you normally would, rather than using them only as a chewable fidget. They are ideal for fidgeting during lectures or for biting when you feel a meltdown coming on.
Here is a link for them:
I do earn a small comishon if brought from this link