Oh yeah, I heard about Mérida. In fact, exactly what you just wrote: simple, relaxed atmosphere, lovely old town center, and extreme heat and humidity. Never been there though, and I think it's worth a visit, despite the heat, which I'm not a big fan of, but it shouldn't kill me. Manzanillo? Sure! I'm thinking I should continue on my cycling journey down the coast: Tijuana, La Paz, Mazatlán, Vallarta, Manzanillo, etc. It just sounds so fantastic!