If all of the universities that I am applying to have a spot to upload my resume, can I just put my activities on there and not fill out the common app activities section?
You should fill out the common app activities section, I think, but you can elaborate on activities further in that resume.
In case a college doesn’t look too closely at that resume, you’ll want to make sure you have some stuff on the actual common app.
You most certainly can. Just put “Refer to resume for activities” or something on the activities section or the additional info section on the Common App.