You're putting some thought into this, which is really sweet. And after only two weeks, she must be special for you to be thinking of her like this.
My suggestion is to forget roses (much too formal for something still relatively casual). And a single rose may carry the nicest thoughts, but depending on the lady she could think it just means you're cheap.
Here is what I recommend. Scrounge up a good sized, but tall/narrow bud vase. If you don't have one around, stop off at thrift shop or dollar store whenever it's convenient. Then... go into any florist anywhere in the country and tell them you only want a few cut flowers - not an arrangement. (This is Valentine's Day, which means the mark-up on anything in a flower shop is horrendous.) Then.... ask them specifically for 3 stems of Stargazer Lilies, a few sprigs of anything colorful (Larkspur, maybe), and a little greenery. Stargazers are available year-round.. they're not expensive unless you buy 2 dozen of them or pay the florist to "arrange" them.. they're bold, colorful, and fragrant.. And most stems will have one bloom that's just opened up, PLUS one or two new buds that will also open into full bloom as the original blossom begins to fade - which means this gift from you will last a long time.
You could also look around where you live for the greenery. Most hedges or shrubs that stay green year-round make excellent greenery if you can cut off a couple of branches that are long enough for your vase. It's the blooming flowers that get the attention, not the green stuff we just stick in there as a background and to hold up the flower stalk. No need for bows, or cards, or ribbons... unless you like that.
So.. it's very simple, elegant, uncommon, and not overly expensive.
Here are some samples I just Googled to give you an idea what this will look like. Mind you, buy one of these ready-made from a florist... it'll cost you about $40-$50. Much more practical to pay top dollar only for the flowers themselves.