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Personally I'd go to the Big Bend Natl Park and go to Boquillas if you really want to go to Mexico. I've been there. It's neat, it's Mexico, and it's safe. There is secure parking and I have spent the night in this town. The Big Bend itself is also way more scenic than anywhere along the lower Rio Grande Valley. According to Google Maps you would be in for 12.5 hours of driving, but if you have 2 days that is perfectly reasonable. I've visited the Big Bend twice and I have enjoyed myself both times I've went. The Santa Elena Canyon is probably the best part of the park. And then there's the mountains and the hot spring.If you cross into Mexico in the lower Rio Grande Valley I would advise against visiting Nuevo Laredo, Reynosa, and Matamoros. All three of those cities are somewhat dangerous and are known for having cartel violence. Larger border cities in general are the more dangerous parts of Mexico. The interior of Mexico is generally much safer. Smaller border towns should be okay however. I've heard of people crossing into Nuevo Progresso. That is probably the safest place to cross in that area. I looked and there is parking on the US side available for $2. Looks like the driving is about 11 hours if you go that route.
Did you end up going to Mexico? Two weeks ago I ended up driving from Ojinaga to Mazatlan and then from Mazatlan to Nuevo Laredo without any issues. I'll definitely not hesitate to drive into Mexico again in the future if I'm ever near the border.
I'd be interested in hearing more about your drive.
Mazatlan has a very large police / military presence which is kind of an eery feeling. I drove around Mazatlan with the windows down however and felt safe enough.
The drive from Monterrey to Nuevo Laredo is considered dangerous by many, but I didn't have any issues and I would do it again without hesitation.
Overall Mexico seems pretty safe. I wasn't at any point fearing for my life or worried about being a victim of violence
The police down there are almost too honest in many ways. They were all very professional, but if you go down to Mexico getting pulled over and getting questioned / searched should almost be expected.
I also arguably drove through some of the so called sketchier areas of Mexico.