I try to be super active in my comments section, but not to the point the point that I respond to everyone~ It tends to come off as not as genuine at that point.
Fan events are also something I do! When I hit 500 subs I let them vote on whether or not they wanted a big drawing, a side comic, or a Q and A event. (I ended up doing all of them, cause I underestimated how long my side comic would take to draw, as it turned into a almost a whole chapter's worth) but they seem to LOVE being able to decide what they receive.
Character polls are also good! It's fun for them to tell you who their favorite characters are, and you can always draw a fun pic of the results. <3