I did 5 internships lasting a total of 20 months at 4 different companies as a software engineer intern and research intern. Each internship felt drastically different and was packed with different opportunities. I'm now a huge advocate of internships and would suggest any student to do multiple.
After my first semester of grad school, I spent over 6 months as a intern at First Tennessee. It gave me the opportunity to work on a number of projects, including fixing bugs in a desktop application, cleaning up a web app, analyzing user data, and working with mainframe applications. I also got to interact with the internal customers of these systems. Even though this work wasn't research-oriented, it certainly made an impact on what was to come.
After presenting my first paper at a conference, I got invited to go to Austin, Texas on very short notice to work at National Instruments. The company provided me tons of freedom to study their engineers and implement my research into their product. The entire experience was different than anything I had seen: people drank beer and played with Legos at work! It went so well, they provided funding for my PhD and gave me remote access until I came back the following summer. During my second summer, I requested to extend my internship and they agreed! Most importantly, Austin has some of the best food I've ever had.
The stars aligned and I took an internship at MSR in Redmond, Washington. My project was ambushed with challenges along the way, but my collaborators and I worked through all of them and the end result was absolutely amazing. The Microsoft campus was overwhelming with endless cool things going on. Seattle is a beautiful city with outdoor advertures in every direction.
Having thought I was done with internships, I opted for one more! Again the stars aligned and I was able to join a colleague's group that focused on machine learning. Since I didn't have any ML experience, it was a different world that enabled me to learn a lot. It made for a fantastic last internship, with a lot of fun times hiking and exploring New York with friends.
I just wish I had another summer so I could go to the San Francisco area! :)