This Weekend: Celebrate the blues by San Francisco's Bay

James Madison lived at Montpelier—an estate in Virginia's Blue Ridge Mountains—for most of his life, yet until recently the fourth president of the U.S. wouldn't have recognized the place. That's because the house changed hands several times after his death, and each owner had a different vision for the mansion. "It was a home, but it wasn't the Madisons' home," says architectural historian Mark Wenger, who was part of Montpelier's recent five-year, $24 million restoration. Today, you can see the property in its original 1820s glory—and watch conservationists at work in the Archaeology Lab. montpelier.org, $14.

It’s no coincidence that rivers run through many of the world’s leading cities. Case in point: London. A new walking tour, the Tides of Time, traces the evolving relationship between the city and the Thames. Docents from Context Travel—a network of archaeologists, historians, food critics, and other specialists—take participants on a 2.2-mile stroll along the river’s South Bank. Beginning at Tower Bridge, the route hits landmarks old and new, such as the Houses of Parliament, the replica of Shakespeare’s Globe Theater, the Tate Modern, and the London Eye. While passing wharves and brick warehouses (some now galleries and trendy lofts), you’ll learn about the river’s industrial role and about efforts to improve its water quality. Just walking the Tides of Time does a small part to tidy up the river; Context Travel donates £5 ($9) from each ticket to Thames 21, a nonprofit that hosts periodic cleanup events. Happen to be in London this Saturday, September 20? You could join a special cleanup day: the Context walk followed by a few hours of collecting trash with Thames 21 members. The Saturday event is part of the nonprofit’s annual Cleaner Thames Challenge (September 18-21, 2008). The three-hour Tides of Time runs regularly on Wednesdays for £35 ($64) per person. No, it’s not cheap, but the walk is a highly curated experience—Context terms it a “seminar,” not a tour—and limited to just six people. Keep up with the company’s like-minded offerings in Paris, Rome, Naples, Florence, Venice, and New York City at contexttravel.com.

If American history class wasn't really your thing (read: you slept through it), here's a chance to get caught up. Washington, D.C. will be hosting more than 80 free walking and bike tours this Saturday and Sunday. Cultural Tourism DC, a nonprofit coalition of cultural and neighborhood organizations, is sponsoring the event. The breadth and wealth of subjects is almost overwhelming (the tours span 18 neighborhoods), but here are a few that stood out: "Art on Line: The Story Behind the Artwork at Metro Stations," "Can You Dig It? The Past, Present, and Future of Mt. Zion Cemetery," and "The National Mall: What the Memorials Don't Tell You," where you can be a guinea pig for a future self-guided tour (plus, it sounds like there might be some National Treasure action.) Meeting points, times, and guides vary - some tours require registration (although all the tours are free). You can get all the info at culturaltourismdc.org.

There are a few things I'm a little obsessed with. Some that fall in the travel category are sightseeing tours, GPS devices (oooo, gadgets!), and those cute Smart Cars. So imagine my delight when I found GoCar, which sort of combines all three. The company hawks GPS-guided tours in San Francisco, Miami, San Diego, Barcelona, and just recently, Lisbon. The bright yellow, three-wheeled cars are available for rent. Start driving, and the GPS device kicks in automatically. The fun comes when the device starts "talking" to you—not only giving directions but also providing a running commentary on the sights that you're passing. Think of it as an overly chatty but very informed local giving you directions and a tour—only you're in the drivers seat. The brand-new Lisbon operation has 10 cars in its fleet, with four available tours. On the East City Center tour, you'll quickly circuit the city, passing such important sites as St. George's Castle. There's also an optional detour into Lisbon's more secluded neighborhoods. And with those painfully cute, two-passenger, open-air cars, there's no telling what conversations you can start with the locals. You can reserve a GoCar in the U.S. in advance on the website or by calling 800/914-6227. For Lisbon, email reservation-pt@gocartours.com or call 011-351/210-965-030. Rates are 25 euros ($35) for the first hour, 20 euros ($28) for the second, and 18 euros ($25) per hour after that. Or, rent a car for the whole day for 89 euros ($126). Rentals are available from 9 a.m. to 3 p.m.