Spent 5 months there on J1 in 2003, so I cant comment on the hotels, but hit many a casino during my stay!
Would agree on O'Sheas, its a bit Oirish with crazy shamrocks and the like but they do metre long cocktails, cheap beer and its good
fun w/o feeling like you need to break the bank. You can find quite cheap gambling around the place if you wander around (e.g. Terribles Casino offers penny gambling machines!).
Of the more upmarket casinos, The Luxor is very plush and has a nightclub called Rain, which is very trendy. Also the Palms has a very cool rooftop bar which might be worth a visit. Ceasars Palace is also quite nice but has expensive gambling as far as I recall. If you are feeling homesick or in need of an irish breakfast, the "Nine Fine Irishmen" (the only Irish bar on the strip as far as I know) is located in the New York, New York. They have live music most nights, and it certainly does pay to be Irish in there!
For free extertainment, the fountains outside the Bellagio are fantastic, especially at night, for not so free I would also recommend the roller-coasters that are on top of the Stratosphere and either MGM/NYNY casinos.
Downtown Vegas, although older, is quite nice. It has an old-world feel, that you dont get on the strip, which is very much new money. Downtown is home to the likes of the Golden Nugget and Binions Horseshoe around Freemont Street.
Grand Canyon/Hoover Dam trip is excellent, helicopter ride over G.C. is amazing. Its quite a long drive to and from, we did it in a day but it is quite tiring, especially if you get adventurous and climb down into the canyon. We went down few hundred feet and its alot harder on the way back up!
Hope you enjoy the stay!