If you want to be within walking distance to Playa del Carmen, you're limited to hotels IN Playa or in Playacar. Playacar is generally walkable to town, although some hotels can be a bit of a hike and many people do take a taxi ($7/one way), especially at night.
Hotels you may want to check into are:
Royal in Playa del Carmen (adults only)
Gran Porto Real
Both of these hotels are IN Playa del Carmen, you can't get any closer.
Playacar Palace - is as close to Playa as you can get and be in Playacar, but I would say this hotel might be a bit more reserved, because it's very small.
Riu Palace (there are two - Palace Mexico and Palace Riviera Maya)
Budget will come into play too - some hotels will be more expensive than others. There are more too, but these are my suggestions.