Piano and classical/flamenco guitar were my instruments back when I studied music..
Guitar is nice, portable, and a great way of meeting girls.. but Piano is an absolute knockout.
Any guy can strum a bit on guitar, sit in a park and impress their mates/girls.. but if you take a girl to play piano, then its something else.
At uni, 3rd/4th date - take the girl up to the music practice studio late at night. Was in a very quiet building, that only arts students usually had the key card to get into (artaud building for you brunel students), the lights were always off, and it was a bit creeping taking her up a tiny staircase and through dark corridors.. But then you arrive, switch on the lights and you see a huge practise room with many pianos, recording stuff, percussion and all sorts of instruments.. Spend a few hours just messing about on the different musical instruments, showing off to her a litt (but not to much that it looks forced)... only did it twice, but both times it just killed it as an unusual date.