Never been that far up the coast, but here are my suggestions...
The easy:
Take a mask, and find some Jack crevall near a rock wall or pier... then chum the water with shrimp to get a feeding frenzy and throw a similar looking fly through there. Those things are hard fighting fish and they seem to be everywhere I looked on that coast. Some were huge and they are not too shy.
The crazy:
I fished near tulum and my do-it-yourself experience fishing in mexico was a bit different. I went to a lagoon about 100 miles south of where you will be (Boca Paila in a national park of some kind - which is beautiful by the way), and fished for smaller bone fish just before sunset. i could tell you the location if you are interested, but keep in mind wading with aligators, LOTSA mosquitos, and we got hasseled by the "park police" and had to give him 10$ to piss off. But it was beautiful in there...
http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=tulum+mexico&sll=22.146708,-96.679687&sspn=10.691938,19.6875&ie=UTF8&ll=20.007528,-87.480826&spn=0.005303,0.009613&t=h&z=17&iwloc=AThe less crazy, but not so easy:
The bridge you see has trails leading in many different directions and you can see big barracuda chasing smaller fish all over the place, so a well placed fly with a steel leader might get you a nice photo.
The ruins in tulum are very, very nice as well, but a bit busy. If I recall correctly, sundays are free days at those sites so they are by far the busiest. If you haven't been to mexico before, I suggest buying your sovenier tequilla at the grocery store, it will save you a lot of money. Also the hamocks and mayan hats are always over priced in the big cities, but thats just part of the fun I guess. Be careful with taxis if you decide to explore the surrounding, as I found it a bit unerving to go 160km/hr with no seatbelt.
Good luck and have a good time.
PS, SPF 50 and don't forget the tops of your feet.