Hi there,
A couple of French-speaking agencies had a good reputation a few years back. They still seem to exist, but I don’t know how their quality has evolved or if the management is still the same:
Toi's Tours: run by Toi, a charming and competent French-speaking Thai woman.
Safarine Tours: a (rather expensive) agency run by a French guy who used to post on this forum and a few others.
Otherwise, Patrick’s right—Kanchanaburi is pretty easy to explore on your own.
Getting there by bus or minivan is straightforward from several bus terminals in Bangkok or other cities in the country (Chiang Mai, Lampang, the Isaan region, etc.), and Banpong’s train station, just an hour away by bus, gives access to southern cities.
All the sights in town are easily reachable, but since the city is quite spread out (e.g., going from the bridge to the JEATH Museum), biking or scootering around will be faster.
Around the city, several sites are accessible by public transport, like Erawan (bus), Phrasat Muang Sing (train), or Sai Yok Noi (bus).
The Wangpo Viaduct (also called the Death Railway bridge or Tham Krasae Viaduct—don’t confuse it with the bridge) is easily reachable by train (which I recommend for the views), but getting back means waiting 3 hours for the next train or walking 5 km (with two-thirds of it being a steep climb) or hitching a ride to Highway 323 to catch a bus back to Kanchanaburi or toward Sai Yok Noi and beyond.
About a dozen kilometers southeast of the city, the photogenic Wat Tham Khao Noi and Wat Tham Seua aren’t accessible by public transport—rent a scooter, take a songthaew, or check with an agency.
Wangpo Viaduct:

Wat Tham Khao Noi and Wat Tham Seua:


