which cities
 

This summer, my family of three teens, 13,16,18, will be going to Mexico for a week! But we aren't sure where to go! They prefer places not saturated with tourists and resorts. We don't want to miss out on beautiful beaches and ruins,but we enjoy a more authentic feel when we travel. This will be our first time and we aren't sure if certain places are right for us. I've seen Puerto Vallarta recommended on multiple sites but I don't know how crowded it'll be. We also will most likely be renting a car and could travel between areas if that is required! Thank you for your help.

No ruins in Puerto Vallarta, but very beautiful with jungle covered mountains coming right down to the sea. Busy? depends on the time of year, high season is from December to April, and, so far this year, not super busy except the two weeks of Christmas and Easter. A lot more information here http://vallartainfo.com

Summer is our rainy season, but it only rains for a couple of hours when it does, and it's warm rain :)

Merida? Valladolid?

Almost all beach areas also attract tourists, and thus resorts are the norm. Summer in PV (and the Yucatan)will be hot, humid, and rainy (sometimes unbearably so), so keep that in mind. Plus, the rain and subsequent runoff makes the waters at PV murky and filled with debris. IMO, the number one place in MX to visit for ruins, museums, music, culture, food & architecture is without a doubt Oaxaca city. It would be really difficult to find the "authentic feel" of MX by sticking to coastal, beach resort areas. Oaxaca attracts enough tourists to make it a worthwhile destination, but not so many that its overrun with foreigners. Plus, at 5200 or so feet asl, the climate is mild in the summer. Good luck.

Yes, we get runoff from the rivers around the bay, but the bay is enormous and the areas affected by runoff are pretty small. BTW, that debris from the jungle, makes for great fishing :)

Agree that the coasts are very hot in summer, and that Oaxaca is an ideal destination. Consider also Guanajuato and the other colonial cities north of Mexico City -- which is itself a wonderful destination.

Tulum would be great, I am sure Oaxaca and other places would be nice, but the OP wants beaches and ruins.