Several Cambridge colleges open their courtyards and grounds to visitors free of charge. Pembroke College is a highlight, with its lovely gardens and chapel — Christopher Wren's first completed building. Trinity Hall and Peterhouse, the university's oldest college, are also free to enter.
Opening times and policies change, so check at the porter's lodge before walking in. Some colleges close to visitors during exam periods (late May to mid-June). Even when a college charges for entry, you can often peek into the main court from the gate.