aside from applying ED lol
If you search “demonstrated interest” on CC you will find many many threads on this topic.
Some ideas offhand are: visiting, interviewing, joining the email list, taking part in online chats given by admission departments (if available), going to any presentation done by a college at your HS or in your area etc.
Also it is worth checking the common data set for schools you are interested in (you can google) to see if demonstrated interest is even a factor for admissions in that school.