Santa Croce, finished in 1442, is the largest Franciscan church in the world and the burial site of Michelangelo. It is also home to one of the most important art collections of any church in Italy. It costs €8 to enter, and you can spend as much time exploring as you would like.
Because both the church and the museum can get very busy, there is an option to book skip-the-line access. These tours usually include a knowledgeable guide, as well!