Hi Des
We were at the Riu Yucatan earlier this year (3rd year running to Playacar) and are already planning to return next year.
There's plenty of excursions of every type to do over there,depends on what you want.
If you take a walk over to the Playacar Plaza (a small shopping area opp the Riu Playacar )5 mins away from the Yucatan, there are 2 kiosks where you can book excursions from a lot cheaper than the T/Os charge(doesn't matter which kiosk you go to as the same guys work on both).
When we went to Coco Bongos in Play-del-Carmen earlier this year, we booked it through the guys at the Plaza who arranged for a taxi to pick us up from the Yucatan reception and get us priority admission to Cocos, from memory it was 650 peso (app $50 US app £35) each and worth every penny.
As a taster here's some of the photos I took around the Yucatan etc this year :-
http://community.webshots.com/user/Roadgiant
If you want any more specific info, PM me or ask on here and I'll try and give you more details.
RG