Interestingly, I recently asked my husband if their were trains across Mexico (he is from Mexico City) and he had no idea. He grew up with the mindset that public transportation was for the poor. But, I googled and found this which is interesting.
Years ago I took a bus in Mexico from Cancun to Chichenitza. I would guess bus service is better now. Still it wasn’t that bad for a few hours. The first class you had a seat, second class was pot luck. But, that was from one tourist spot to another. From Mexico City it might be less appealing? Your friend should know. Or, he might have no idea like my husband. :)
I know many people who like Costa Rica. Or, what about Belize? I worked with someone who grew up there. If you don’t speak Spanish that can be a nice break in a language you are familiar. Although, many people in Mexico will speak English.
I actually like Cancun for a little beach, air conditioned sand that is never hot on your feet, and playa del Carmen just south of Cancun is less touristy feeling. Cabo is supposed to be great on the Pacific, and also Puerto Vallarta I hear is very nice to visit, less touristy than Cancun. If you like ruins and history there is a lot of that. As I mentioned above Chichenitza has Mayan Ruins. In Mexico City there is a large museum, I think it is called the Archeological museum? Not sure? Here is a website for Mexican tourism Mexico has everything from rock climbing, rafting, beach, city. Not sure what appeals to you.
I personally would be afraid to drive. The police force is corrupt in most of those countries. In the beach resorts in Mexico the local bus system is the best way to go and taxis.