I was pretty skeptical of going to Sugarloaf, but I have to admit - you have to go up there if you're going to Rio.
We went on a cloudy day and didn't wait in line at all. Ever. It was really nice not to have to wait, when our friend (a Carioca) says that the line can be over 2 hours long, just waiting to buy tickets and take the first cable car.
Make sure you bring a jacket and wear comfortable shoes, because you can hike around the back of the touristy areas at the top and get some great photos away from the other tourists. The wind at the top of the highest peak is incredible, so even if it's warm down on the beach, bring a jacket or hoodie with you if you plan to go up there.
Do bring some money with you to eat at the cafes, get an açai & granola cup, or buy some souvenirs at the shops.
While it was cloudy, we did manage to get some good photos from the first and second peaks and it's well worth the amazing views.