It's funny you guys mention Oaxaca and chains in the road.I wrote a long winded reply to this post yesterday that I accidently lost before posting.I was advising against driving at night because a lot of weird stuff goes on in the middle of the night in Mexico,like indigenas putting logs across the road.I have ridden many night buses and always wonder why so many people are out at 3 or 4 AM in the middle of nowhere.The indigenas move the logs if they approve of you passing..In Guerrero in the late 70' s Indians used to just get in the road to block traffic and try to hustle tourists and ricos from Mexico City headed to Acupulco.Their would be little old ladies under 5 feet tall with a 4 or 5 foot iguana on their heads trying to get you to take pictures and pay for them.
The people with the logs were in Oaxaca.Which route did you take Dan? We went the long way via Salina Cruz a boomerang shaped 9 hours to Puerto Escondido.I really like Oaxaca City.Chiapas and the road to Chetumal had a lot of military checkpoints .
Others already said most of the advice I'd give.I would agree with Brian ,it would be a shame to blow by the Valles area.Aside from the standard stuff there check out the Minas Viejas run ,really looks cool..You could also go a half day past Veracruz to the Teapa or Palenque areas.Checkout Sierrarios or Mayanwhitewater for info on the Almandro,LaSierra ,Tulija,AguaAzul,and many others in the Tobasco-east Chiapas zone.
Also hope you'll take the opportunity to see off the river attractions like :
Ruins -El Tajin and Cempoala are on your route.El Caujilote and La Vega can be visited by boat on the Filo -Bobos(3+) down river from Tlapocoyan.Palenque is a premier Mayan site and close to Agua Azul.
Natural wonders-Puente Diablo and Cascada Tamul in SLP...Pico Orizaba and El Encanto waterfall in Veracruz...Macuspana and Agua Azul in Tob.\Chi.
Beaches-Not on par with the Pacific or the Caribbean,,but a lot better than Colorado's ocean
Tuxtla and Chachalacas are among the best.No need for no tell motels I had a room for $8 at a dilapidated beach motel at Barra de Nautla (mouth of Alsaseca -Filo Bobos) no frills and lizards on the wall.
Other attractions-Xilitla,Veracruz zocalo,Spanish Fort San Juan Ulua in Veracruz,2nd best archeological museum in Mexico in Jalapa,and it is only apx 4 hr.. $50-$75 bus to Mexico City.
Old school Mexican boating on the Antigua,the Arkansas of Mexico,contiguous intermediate runs around Jalcomulco with hot springs at Banos Carrizal also a put in or take out.Boats and shuttles readily available.
Have fun .Eat at places popular with middle class locals.Mexico is a gastronomic paradise.Try unusual foods.Ummm,chilaquiles de pollo for breakfast and chapulines for snacks.