The harp, like the piano is a kinda complicated thing to record.
I'd stay away from lavs or any kind of contact mics.
The traditional technique is a stand mounted condensor mic placed at least as far away from the instrument as the instrument is tall.
The space around the harp is also important. A quiet sound field and a space large enough to let the instrument sounds intermix prior to getting to the mic will get you the best results.
Do a web search under "micing a harp" and you should find some manufacturers recommendations.
I think the traditional mics are either large diaphram condensers like the AKG-414 or small diaphram condensors like those from Schoeps or Audio Technica.
Just bring a way to monitor what you're recording (decent headphones!) and use the best gear you can get your hands on without going too nuts.
Really, there are proabably a couple hundred microphones from many manufacturers and models that can do a workmanlike job of recording this. And if you can't get anything but a Shure SM-57 don't let that stop you. Just move it around until the lower end and upper end are sorta in balance and toss a heavy blanket over the refrigerator or anything else making noise in the background and do the best you can.