Tours, shows, exhibits, and other activities are being booked by some travelers farther in advance than ever. The reason: Internet travel sites, in an effort to distinguish themselves from each other, have been adding activity and excursion options to the menu of products they sell. For example, these days many websites, such as Travelocity, allow you to book a double-decker bus tour of New York City at the same time that you book your airfare and hotel room. Generally speaking, it has become easier to plan vacation activities in advance, and more and more travelers are doing it.

Eventually, an increase in the number of early bookings will make it more difficult for spontaneous travelers to book activities on the fly. Luckily, that's not a problem yet.

Case in point: I recently chatted about this trend with Michel Barraud of Paris Vision, which runs more sightseeing tours in the capital of France than any other company. (Paris Vision is such a large company that the chances are high that you're handing money over to them if you do any sightseeing in the City of Light, such as a nighttime cruise of the Seine.)

Michel sketched out the overall trend. In 1999, his company opened its website with booking and payment options. In its first year, only about 3 percent of reservations came through the Internet. But in the past two years, sales have doubled to about eight percent. More importantly, advance bookings coming through partner websites and agencies, such as Expedia, have also doubled. About half of Paris Vision's bookings now come from these independent operators.

How early are customers booking? Michel says that about 20 percent of customers book Paris Vision's tours and other activities more than 45 days in advance. Another 30 percent of travelers book between 15 and 45 days ahead. So half of travelers are booking less than a week ahead.

Michel recommends Viator.com as an online source for booking activities and tours. The site offers reservations for activities from multiple suppliers and for multiple destinations worldwide, giving the broadest range of options available. (Not surprisingly, Paris Vision supplies tours to Viator. But I'm still impressed enough with Viator that I encourage you to take a look at it before you plan your next trip. You may find that some of the activities available are so compelling--such as a sunrise tour of a red rock dome in the Australian outback for $100--that you'll want to plan your trip around them, rather than the more customary practice of booking airfare and hotel before booking activities.