Senior Year Activities & Forgotten Activities

So I asked one of my admissions officers about updating them with new information that happened after I submitted my initial application, and they said that I could, but I was wondering what happens if I forgot to put some activities/positions in my initial application?

Also, how important are senior year activities? I started doing a lot of volunteering and work centered more around the majors I applied for because I want to get more experience in those fields. Will it hurt my chances if I notify my admissions counselors of these activities since it could just look like I’m trying to beef up my application?

Thanks for the help