I recently interned at Microsoft, and I was able to get a return offer for another internship for the same team. I'm back at school now, and my goal is to nail the return internship so I can get a return offer to work full-time. In my spare time while at school, how can I best prepare myself to hit the ground running for my return internship? For context, I worked with Azure APIs during my internship, being more of a back-end developer.
It is amazing to see your enthusiasm. Without knowing exactly what you worked on etc, I suggest reach back to the team and ask them - they will be able to guide you best.
TBH, there is a decent possibility you do something and it doesn't help at all. Internship projects are usually not that difficult, so ask them what you can do and do that. Try to decrease the amount you do that doesn't lead to anything meaningful - something to be aware of in opportunity cost, would you focusing more on your college grade be better use of time.
I don't think there's much difference in how to prepare whether it's your 1st or 2nd internship at Microsoft. The only difference I can think of is that the bar will be higher if you're trying to convert to full-time after the upcoming internship (your previous one was converting to another internship).
When you find out which team you'll be on, I'd do some research in that domain and try to build some toy apps in that area. Other than that, could you ask your former intern manager for feedback?
They'll have the most actionable (and hopefully candid) advice on what you should do.