pea shooter + rohypnol
requires a bit of practise and works best in bars / clubs on split levels, you could just stand directly above and drop it in but that makes for good cctv coverage, best to hone your skills so that from a good 20 paces away and at an angle of 18degs or more you can accurately deposit the "package" securely.
then my friend a casual stroll to the bar, order a tall cold one before heading back and asking if she feels ok "you look a bit peaky hun, perhaps some air would help"
result, and it avoids the need for handing out a fake number etc