What if you don't want to register for wedding gifts, but would rather have a money tree or something similar. Is there a tactful way to do that?

There just isn't a way to request money with out being tacky. If guests ask what you would like, you can drop hints, but cannot directly tell them what to give. Hints that cash would be preferred always start with, "We really have all the household things we need, and we're just happy to have you included in our celebration… but, we're saving up for…" Guests who are comfortable giving money as a gift will figure out that a contribution to your savings project is really what you would like. Some guests simply never give cash! So you should always give a real project that you are saving for, like your honeymoon trip, in case a guest decides to get you something you can use when you finally spend the savings on the stated project. It may be easier for your family to pass on the word that what you really want is money, however they can use the same tact of "saving up for" and should not directly request money.