How long you going to Cancun for?
Well to start off, here is a map of Cancun
http://www.solartours.com/ShareWeb/Maps/Mexico/Cancun%20Map.gif
Ive been to Cancun twice - once for my spring break in 03 and once with my gf's family. The First time I stayed at Oasis Viva which was called Calinda Viva at the time. It was a nice hotel, but it wasnt anything spectacular..just a basic hotel. Second time I stayed at the Royal Mayan (its not shown on the map for some reason but its around the Del Rey Ruins and Hilton...maybe it changed names to Omni since there were three Royal hotels right next to each other - Mayan, Islander, and something else). The Royal resorts were extremely nice - 5 star Id say. But the beach by the resorts wasnt all that good. All the glamorous resorts are on the longer vertical side of the inverted and flipped L, and the smaller, more homely hotels are on the horizontal side. But the beaches are better on the shorter side as it faces the bay and is shallower and calmer. As opposed to the longer side which faces the Atlantic Ocean and falls off sharply and has crazy rip tides. The beaches are also wider on the shorter side.
From the looks of it, your hotel is right at the tip of the L which is a very good spot since the area around the convention center is the hubbub of the strip. There are a couple malls right there, lots of bars, plenty of places to eat from the local smaller joints to places like Hard Rock Cafe, and some clubs like Coco Bongo, DaddyO's. When we stayed at Calinda Viva we just walked everywhere (except far away places like Senor Frogs which is closer to AquaWorld) but we had to take a bus into happening place when we stayed at Royal Mayan. Speaking of which, you dont need to rent a car there. Buses run up and down the strip (its just one road going up and down the L) from early in the morning to 2am. Buses were 10 pesos/ride which is nothing and the buses run every 15 mins or so. Public transportation is much better than in Aruba (the place is generally more fun than Aruba, but not quite as relaxing).
Some things I'd recommend
Food
Senor Frogs - fun filled festive dining. hot waitresses and the food's pretty good
Also there are some fabulous street places in the bazaar right across from the Convention center. Two places we went to were
Beef Fajita Tacos and
El Tacolote. Best food I've had in cancun. In fact I wanna go back there just to eat at those places
Clubs
Coco Bongo - must see. they have a circue de soleil type show every night right above the huge dance floor. pretty freaking cool. Worth the cover to get in.
La Boom - dunno if this place is still around or not, but I saw the best wet t contest at this place during our spring break trip. might be calmer in non sb times.
Activities
Jungle Tour - you follow the tour guide out into the ocean in your own boat for about 30 mins, snorkel for 40 mins, then take boat back. We did Jungle Tour each time we went but did it with different tour agencies each time. Forget the name of the first place but it left from the shorter side - boat ride was longer but the snorkel site they took us to wasnt as good as the 2nd time. 2nd time we booked with Chaza travel (i think thats what it was called) and they left from Aquaworld IIRC. Their boats were better and the snorkel site had more fish too (but snorkeling at Cancun doesnt hold a candle to that at Aruba...but its fun nonetheless because of the boat ride)
Isla Mujeres - its a small island north of Cancun (you can see it light blue in the map). Hit up this place. Its not as crowded as Cancun and if your girl wants to do a little shopping, she can get some good deals (after bargaining) for some cool stuff at the marketplace there. Also Playa Del North at the north west tip of isla mujeres is the best beach there. The entire north face of the island is pretty rough and its fun walking along the "boardwalk" there. You can either go with a guided tour or take a ferry from the short side. We went with a tour (chaza travel..left from Aquaworld) only because we got some promotional deal and it was free for us. But the tour was a riot. The guides were all singing and dancing and serving liquor on the ferry ride there and back.
Xcaret - If nothing else, I'd recommend you book a tour to this place and check this out. Its about 1.5 hrs south of Cancun (not the Xcaret Paradise you see on the map i linked) and lots of places have tours to this place. Xcaret is like a natural park but the best part is the underground rivers. very very very cool. Definitely check this place out. Make sure you bargain between tour dealers. Our first quote was $67, but after talking to three or four tour dealers (they all work for the same company btw) and playing them against each other, we got it down to $42 which included transportation costs and park entrance fees. You might get lucky. Its an all day ordeal. Buses come pick you up from your hotel at 6am and you get to Xcaret around 10 am (even though they come pick u up at 6, they dont leave their central station in Cancun until like 8:30). You have all day in Xcaret to do whatever u want - check out the birds/animals there; go for a swim in the underground rivers; or just lay around at the lagoon. There are two returning trips - 6pm and 10pm. take the 10pm trip back because you definitely dont want to miss the night show - words cant describe how awesome this show is.
Chichen Itza/Tulum - Mayan Ruins. Chichen Itza is the one we visited during spring break. Cool place and lots of photo ops in front of the huge pyramids but if its down to between this or Xcaret, Id pick Xcaret hands down.
Well thats all I can think of for now. Hopefully I have answered all your questions and more. Let me know if you want to know something else. And I'll send you a PM if I think of something else.