Deciding to do "away rotations" at another institution is always a big headache for 3rd year med students. Should you do them? Where should you go? Will it be helpful? What should you do there? etc. etc.
Should you do them? - If you are applying to a competitive field, I would strongly recommend you do at least 1 away elective. For something super competitive like Derm, I would recommend at least 2. For less competitive fields, away rotations are optional.
Where should you go? - Only do away rotations at institutions that you could easily see yourself at for residency. Away rotations are "auditions", but at the same time, it is for you to decide if you want to do residency there. I would also choose to do an away rotation at an institution in which I would be considered underqualified (by step scores, etc.). This can get your foot in the door at places your foot couldn't have gone before. Also if I am already fully qualified to get an interview at a program, I don't gain much by doing an away rotation. In fact, an away rotation could hurt if I don't get along with residents, don't put enough work in, slack off, etc. etc. Analyze whether you have more to gain or more to lose before choosing an away rotation.
Will it be helpful? - There is an unwritten rule that most students that do an away rotation will get an interview. At some places, not rotating there means no interview (very selective places). If you have less than stellar scores and want to do residency at Harvard, DO an away rotation there. Away rotations can also be helpful if you are really considering a location and just want to see how the program works. Still, you need to work hard.
What should you do there? - While there, I would strongly recommend setting up a meeting with the residency program director. Let them know you are interested. Come early to the rotation and stay late. Offer to help with anyone's research project. Be a team player. In other words, plan to work harder than you have ever worked. Your hard work could get you into a residency that you otherwise would never be able to obtain.
Don't feel pressured into doing away rotations, because everyone else is doing it. Many people don't do away rotations at all, and they do just fine.