Everything! I got hands on experience and got to work with a lot of incredible people. I learned so much and I trust my engineering skills so much more because of it.
What I wish was different
There were occasionally days where I wasn't doing much, but that was simply because there wasn't work an intern could do.
Advice
When you don't have anything to do you can make your own work! I helped to organize the lab, for example. The great thing about this internship is that you can make it what you want it to be.