The official tourism office maintains a pretty up-to-date website at https://hellosaintlau.fr//.
Buses from Cayenne and Kourou take you to Iracoubo, from where there are buses to Saint-Laurent. There are also direct minibuses that don't run on fixed schedules. A minibus ride will cost you around €35.
Route Nationale 1 from Cayenne ends in Saint-Laurent. From the Surinamese side you can take your car with you on the ferry.
The town isn't particularly large and you can get everywhere by foot.
The Camp de la Transportation on rue Tanon is Saint-Laurent's reason for being. It's where prisoners (including the famous Papillon) arrived for processing. Guided tours only (90 minutes, €9). Most guides speak some English. The tour includes the cells and public execution areas.
There are also other buildings from that age - in the 19th and early 20th century the region was first and foremost a penal colony.
Rent a canoe from Maroni Club on Esplanade Baudin Tel 23-52-51.
Tropic-Cata give boat tours Tel 34-25-18.
- Gîte Amazonie Accueil, 3, rue Rolland Barrat, ☏ , firstname.lastname@example.org. Wi-fi, shower, laundry machine. 10€ for a hammock, 15€ for a hammock with mosquito net.
Most visitors go either west to Suriname or east, further into French Guiana. Another alternative is to travel upstream by boat. 17 km south you will find another penal colony, Saint-Jean. 50 km upstream there's the village of Apatou and a further 20 km south there are the Voltaire Falls.