Keep in mind that Brazil is as large as the continental United States, so there's no way to give all the possible nice places to visit in the space available here. There are several good guide books available (e.g., Lonely Planet guide to Brazil). Flights aren't cheap, so distances and available time are important considerations. You don't give any details regarding how much time you'd have, what you want to do, what places you may already have in mind (why are you going to Brazil, anyway?), budget factors, etc. For flights you might consider looking into getting a Brazil air pass, which you'd need to buy before you leave your home country.

For seasons, it does of course make a difference exactly where you're going. We're in the northeast (Fortaleza) where the temps don't vary alot throughout the year, though there is usually more rain Jan-March or April. In the south, including Rio and Sao Paulo, but especially south of there, it can get a bit cold in the winter. If you plan to go to the Pantanal or Amazon, you may want to avoid the rainier seasons (guide books will have details). There are seasons in terms of costs, as well. I couldn't tell you all of the possible vacation periods, but I can say that the peak times for international visitors, in terms of numbers of tourists and costs, are New Years and Carnaval. In Rio, particularly, you will run into higher than normal rates, and minimum stay requirements, both for New Year's Eve and Carnaval. If you really want to be there for one of these events, be prepared. In general I'd suggest going in "slower" seasons, such as after Carnaval, October, etc.