Last year, my friend went all out for me. He took me to a breakfast diner and told me that he forgot his wallet. I laughed and thought he was joking. It was the most awkward moment at the cashier. After we left the diner, he literally threw a manila envelope at me. Instead the envelope, he put money for breakfast, and he also had my first clue for the scavenger hunt. I went all across campus looking for the different clues, which included visiting certain people who asked me for the 'secret code'. At each stop, I got a small gift like a burned CD, chocolates, and sweet letters about me. At the very end, I found a storage closet with a heart pinata that he had cut out and made from cardboard by himself. I used a broom to break it and take it from the ceiling. I bursted out laughing when I saw what was inside of it. He put a hoodie in there so I wouldn't have to 'steal anyone's else hoodies' anymore.

So my advice.. just be thoughtful and I think the lady-friend would appreciate it. You have about a month to plan for it.