An easy one that no one's mentioned yet that's mutually rewarding: volunteering.

I've volunteered all throughout Uni, at numerous organizations related to my major. The longest one I volunteered at was for a year and a half, and it was a charity that visited children who were sick with cancer or life-threatening diseases in the hospital, and also setup a free camp for them every Summer through fund-raising. Honestly, it was fun and very rewarding to know I was helping make those kids feel better.

For some of the months, I was in the office logging checks and recording donations, others I was at donation events or events hosted for children, setting them up and if they were for the kids, playing with them (once a week we'd have a large room on a hospital floor in the children's hospital, we'd play video and board games with the kids who were feeling well enough to participate). On those days, it only took an hour to two of my time. The office days, I stayed for up to 6 hours sometimes for free.

But they literally wrote me an amazing page and half letter of recommendation that you can tell is not copy and pasted and offered to speak on my behalf at any time in the future on it, and this place including all the others I volunteered at look great on my resume while I'm finishing uni. People always ask "how am I supposed to get two years of experience for an entry-level job out of college?" - this is one way.