You can! It's not trivial, but not very hard either.
eye tracking - OS projects getting this from a laptop camera exist and work well enough for assistive tech (i.e. it can replace your mouse today)
electrical activity in the brain - there are fancy things like https://zeto-inc.com/device/ but there are basic options as well
heart beats and rhythms, muscle activity, blood pressure, skin conductance - there's lots of cheap sensors for those