The National Maritime Museum has many things from the world travels of Captain Cook and Lord Nelson. They have a small replica of the Beagle, and some history of Charles Darwin's research at the Galapagos. On my last visit there, they had a replica of the James Cairn (used to make the movie), the boat that Ernest Shakleton and his five men took to South Georgia, a 8,000 odessy.
Greenwich also has some nice restaurants and shops.