The Microsoft intern program is well-organized with lots of special intern events (eg game nights, Mariners games, hiking etc). You get lots of perks with your salary including subsidized corporate housing, gym membership, and lots of gifts/swag.
Microsoft is large and fragmented, so your experience will depend a lot on your specific team and manager. Overall, the people are smart, the culture is laid back, and the company mood is optimistic.
There is lots of interesting stuff going on at Microsoft, but 4 months is not enough to ramp up and fully take advantage of everything. Your internship will likely be project-based and you shouldn't expect your intern project to be something cutting-edge.
Return offers are relatively easy to get as long as you put your work in.