We just finished From The Mixed-Up files of Mrs. Basil E. Frankenweiler by E.L. Konisburg (http://www.amazon.com/Mixed-Up-Files...=UTF8&s=books).
I LOVED and was completely captivated by this book as a kid, and I was pleasantly suprised as I read it to my homeschooled kids. The main characters, brother and sister, Jamie and Claudia, run away from home to live at the Museum of Modern Art. Originally, Claudia, the mastermind and planner, hatches the plan to run away because of the sexism in her family, the unfair workload and the general monotony of her life. She invites her brother Jamie along, motivated primarily by his large sum of saved money (about $23). The tale ensues about their survival and then becoming interested by the mystery of a recent museum acquisition of an angel sculpture and whether Michalengelo was the sculptor.
What was impressive to me were the ideas of the kids learning on their own (following school groups, choosing different areas of the museum to learn about) but also going to the public library to research Michelangelo. I also loved how the brother and sister negotiated and worked together.
I would say this is about a 7-12 year old book, but I am sure all ages would love it.