Sumida River, Tokyo
After visiting Hama Rikyu Gardens (a 10 minute walk S. of Tsukiji Fish Market), you can take about a 40 minute ride on the Sumida River to the pier near Asakusa. Cost: 620 yen (one way). It makes for a nice change in pace and you get to sit down and see the areas along the river.
Caution: There are a few low clearances under a few of the bridges if you're on the top deck of the boat!