In general, the basic rule is "things you had on your person" at the time of being pulled into the world. So gald, swords, food, etc. are all likely candidates. More magical items that heal such as gels will have their consistency changed to be spirit water in gel form. So they'll still heal the same amount, but the reason for their healing will be different. Try to limit it to around 20 gels/healing items total and save them for a rainy day! And you should probably find a waterbender healer for the best application. Life bottles can be the purest of spirit water, but they can only revive unconsciousness and not death.

Magical items can still be brought, but they won't work. Like the Triforce from Zelda, for example. It could be in the game but it wouldn't work at all. Just a bunch of triangle shaped pieces.