I second Joan's suggestion about London Walks and Paris Walks. We have enjoyed walks in both cities and thought them fun, informative, good value and easy - no reservation needed. Rain or shine.

Paris: http://tinyurl.com/2wowd We did Hemingway's Paris - Hemingway and many other writers.

London: http://london.walks.com/ We did a pub walk - the Old Knightsbridge Village Pub Walk.

The site has links to other similar walks' companies.

Frances Toronto, Canada