The Mariscal Sucre area, or "Gringolandia" as it has become known, is the main tourist hub of Quito. It runs from Parque El Ejido in the South up to Avenida Orellana in the North, and is centered around the roads Juan Leon Mera and Reina Victoria. Most of Quito's budget accommodation is located here, as well as every tourist service you could possibly want.
A five-minute walk through this area will take you past countless restaurants featuring every cuisine under the sun. It also features popular bars and clubs that stay open until dawn, as well as numerous internet cafes and other basic services like banks, laundromats and mini-marts. Here, you will also find Quito's best bookstores, LibriMundi and Confederate Books (which features books in English).
Most Quito travel agencies also have their offices here, so it's a good place to find a tour, whether you're after an international airline ticket, a week's cruise in the Galapagos or a mountain-biking trip. Quality and prices vary a lot, so shop around.