We've been to Mexico twice and stayed at the Grand Sirenis and Sandos Playacar. Both are very nice hotels with beautiful rooms with double jacuzzi baths and seperate showers. Sandos (and actually any of the hotels in Playacar) is within walking distance of Playa Del Carmen, which is a great place to wander round or go for a meal/few beers.
Playacar itself is also nice and there's a small shopping complex there with a Starbucks and there's also a bird centre.
I wouldn't pay for an organised trip to snorkel with turtles in Akumal. You can get a taxi or Collectivo (local bus type thing) to Akumal beach and then hire snorkelling equipment and a locker on the beach. Much cheaper and you can take your time too.
If you like Monkeys there's a place called 'The Jungle Place' which is a sanctuary where you can go in with the monkeys and interact with them. Tour operators don't offer this trip and its really popular, so you need to book direct and well in advance.
I love Mexico. It's very commercialised compared to the rest of the Carribean, but not horribly so IMO.
Oh, we've been in November and December and the weather was great. A bit cool compared to other Carribean destinations, but not chilly. October is more the end of the hurricane season really. Although anywhere tropical means you'll probably get some rain but you'd be unlucky for it to ruin your holiday.