One of the most famous things to see when you come to Greenwich, London is the Cutty Sark, the last of the tea clippers to sail between China and England in the 19th century. Unfortuantely, right now she's under a huge white covering (see photo) as she's being given a long overdue makeover and will be in excellent condition by late 2008. However, you can take hard-hat tours focusing on preservation techniques.
You can still enjoy the Cutty Sark Gardens surrounding it (but don't expect grass, just cement flooring and a great view of the river) where you can happily allow the kids to feed the most obliging pigeons. It is well worth a visit this spring break!