I think he meant a marriage proposal but that's clearly not serious.
Take her out for a day or something. How much are you looking at spending? What are her hobbies and interests? Not all girls like the same thing so more information would be useful.
something simple, dinner.. don't go all out buying presents, you would hardly know her only being with her for 3 months. She mightn't like what you get, but who doesn't like a nice dinner?