Find out what she likes best, using Saran Wrap only as a last resort.
My partner says that the best way to impress a girl is to wrap yourself in nothing but Saran Wrap and say, "You're the big winner and I'm your prize!" This is why my partner does not write for Love and Learn and I do.
Clearly, the most important step is to find out what she likes best. Lots of us like to get flowers, but that's not for everyone - and it can be expensive. What impresses me most is when I get the (free!) gift of time - help me finish a chore or take an extra turn making dinner just so I can have some extra time. An amateur massage is great (and sets a romantic mood). Tell her you love her. Make a fuss over her by bringing a homemade lunch to work. Again, the key is finding out what she likes most, then doing it.
I bet our readers can help out by telling us things that have impressed them the most.