Oh Linz! You will LOVE it! I've been 3 times and I'm only 20, not 21 and managed to have a fabulous time!
Where are you staying? (Sorry if you posted this already, I just got so excited to give you suggestions!)
The first time my Fiance and I went, we stayed at The Venetian. Definitely check it out! Try to swim in their pools, sometimes there are no guards! They are fabulous!! The Venetian is definitely one of my favorites! It is absolutely gorgeous, with paintings on the roof, gold accents, you name it! Check out Madame Tussauds wax museum, it's in The Venetian. (There are coupon books at the monorail that have a 2 for 1 coupon usually..or something like it so make sure you check there first to save money!)
I also loved the water show at The Bellagio! It is amazing and unlike anything you've ever seen before. I think it's every half an hour, I could be wrong though. They play different music all the time! MUST see!!
Caesar's Palace was amazing to see. I felt like I was in Rome!! I love going there! Go to the forum shops there, check out all the ritzy shops. Also, I just love this little shop called "Vosges"
Luxury Chocolate Truffles, Gourmet Gifts & Specialty Recipes at Vosges Haut-Chocolat
They have the most amazing chocolate! If you like chocolate, you must go there! They are usually giving free samples too. You can also get really cool chocolate drinks there too!
We ate at Jimmy Buffet's Margaritaville which is right across the street from Caesar's Palace. It was great food, and actually a good price (which is odd for Vegas!)
I would go to Fremont Street again to check out the light show there. It's this amazing light show on the roof - every hour I believe! it's only a few min taxi ride to get there. Cheap souvenirs there too!
You can shop til you drop in Vegas, but remember the exchange rate! You can usually find some great sales, and cool stores that we don't have here though! So enjoy it!
Go to the New York Pretzel Co. stand in New York New York hotel and get a pretzel. They are all so amazing! They also have the most amazing gourmet candy apples! Can you tell I love junk food??? haha!
We drove there two of the times we went, so we went to a lot of the outlet malls far away, but there is one that is pretty close to the strip.. Las Vegas Outlet Centre is what it's called.
That's all I can think of for now!! ENJOY and have a blast! I wish I were going back!! I will add more if I can think of anything! I'm sure I will!!