I recommend starting the activities section after your personal statement. Again the best place for information on this section is straight from the source the AAMC Guide:
You can write about up to 15 activities and 3 you pick to be your most meaningful and will have extra space to talk about. When you are writing these remember that the person reading is looking at tons of these so chances are your role is not super unique, but that’s okay! Still share what you do in your role but you can also share a story(hard to do in the short space) or share what you got out of the experiences. Make sure you reflect and think about the experience so you're not saying something superficial or cliché. You want to show in the description how you have qualities that will make them want you at your school. Things like leadership, teamwork, growth, and all the AAMC core competencies. I typically included why the activity was important to me. This can all help make it more personal. Keep in mind you do not have a lot of space so avoid repeating information. When you enter activities you fill in the experience name, organization name, dates, and other information. That is all information you do not need to repeat in the main description space.
All the experiences you have 700 characters to write about it. For the most meaningful experiences, you have an additional separate space to write 1325 characters. This is enough space to tell a short story or go into more detail about what you got out of the experience. I personally didn’t use stories except for one small one and was still interviewed at 7 schools. I think what really helped was making sure my descriptions showed different core competencies, especially leadership. Emphasis on the show not tell. I described the leadership task and the role I played in different groups even when I didn't have a leadership title. There are different ways to approach it.
Finally, your activities do not have to be science-related or an organized group! If you have a passion for something like painting, dancing, or a blog you can definitely include it. This is a great way to show medical school who you are as a well-rounded person. Medical schools appreciate the passion and time you put into other things while balancing school and research stuff so don't forget to include it if you have space!
Include: what you learned and got out of an experience, not just your role/ everyday task
Don’t repeat information already entered like your title
Focus on showing your qualities in your descriptions