It depends how long you have been together. Never buy something cheap. A classical gift contains pralines and flowers, take her out to a good restaurant and then have in addition a gift like her favored expensive parfume or some jewellery.
If you do not know her ring size, then think of a necklace, a brooch, a bracelet or something like that. You do not have to spend thousands of Dollars, but also not just 50.
Jewellery is something that last forever and if it is nice then every day she will remember it.
And if she had before you already friends, then she will also compare your gift with what she received before and then shew knows that you are different and may Mr. Right for her.