Brazil - Access to Brazil is via Lethem. There are Interserv buses - get the schedule at the Interserv Bus Office located on Charlotte Street in downtown Georgetown. The buses typically leave late at night. Another option are minibuses that ply the Georgetown-Lethem road, although the lack of paved road beyond Linden means that the trip will probably need to be broken up overnight.
Suriname is reachable via minibuses & a ferry, or by a short flights from Cheddi Jagan Temeri International airport or Ogle airport.
Trinidad and the rest of the Lesser Antilles is a short flight away via Caribbean Airlines or LIAT.
Venezuela There is a walkable track from Venezuela from Kamarang if you walk with enough food, and there are boats that go up the Cuyuni past Ankoko Island and Ekiteringbang if you ask the petrol dealers in Bartica.