Going to Copán? (You should... it's one of the best sites.) There are two ways to get there. The more popular route is to fly into San Pedro Sula in the Honduras, and rent a car or take a bus to Copán (which is almost right on the border with Guatemala). The other way (which few people do) is to fly to Guatemala City and rent a car, and drive halfway across Guatemala on Highways CA9 and CA10, crossing the border into Honduras at El Florido. We choose the second method (mainly because the flights into San Pedro Sula did not fit into our schedule). On the day we crossed the border we were the only Americans to do so, and when we returned two days later, our paperwork was still on the top of the stack pinned to the Customs wall.
It was quite an experience to rent a car in a foreign country, drive halfway across that country, and cross the border into a different country. Looking back on it, it was a lot more audacious a stunt than I would normally have attempted, though at the time it seemed perfectly reasonable. Luckily, we encountered no problems. I was thankful, however, that we paid the extra money to upgrade the rent-a-car to a big Chevy Suburban. The vehicle's size and expense helped us bully our way through the traffic in Guatemala City, and garnered us respect from the locals that we met in the roads and towns. The ruggedness of the Suburban also made it easier to travel the unpaved 50 kilometers of road near the border crossing, especially when it came to fording the half dozen streams that the road crosses on it's way. (We crossed during the height of the dry season, which meant that the streams weren't too deep.)