according to my pre-med advisor it really depends on what the activity was. if it is something standard, like dean's list, or emt training, you don't really need to write anything (but maybe where you did your clinical hours for emt training). if it is something pretty unique, or something a lot of applicants won't have, or something that medical schools likely won't be familiar with, you should describe what you did with 4-5 sentences and also describe what you got out of the experience (I learned x skill, or this experience taught me x about what i want to do). Basically, if you are going to use most of the space you are given, make sure what you did is interesting enough that the adcoms will want to read it, or they will be annoyed that it's long.