You could actually easily spend the entire day indoors if you were so inclined. Many of the Keikyu trains continue through to the Toei Asakusa subway line, which has a few stations connected underground to a number of interesting buildings. Higashi-Ginza Station in particular is in one corner of a massive grid of tunnels that runs all the way to the far side of Tokyo Station (clearly visible if you zoom in on Google Maps) and you can walk around for several km down there, popping up to look around inside the Ginza department stores, Tokyo International Forum, Palace Hotel, Bic Camera Yurakucho, numerous mall-like office building spaces in Marunouchi, etc. Another option is to ride up to Oshiage and visit Tokyo Sky Tree.