Although prenuptial agreements give you a lot of freedom to determine your own terms for ending your marriage, they don't give you carte blanche to override the law. Hawaii's adoption of the UPAA doesn't permit you to address issues regarding your children. You can't use a prenup to waive or determine an amount of child support, nor can you use one to decide custody terms if you should ever break up. Beyond this, however, you can make certain determinations that can override what a court would do if you were ever to divorce by trial. You can decide what property you would like to share in the event of a divorce, including future earnings or property you haven't actually acquired yet. You can waive the right to alimony. You can also waive your right to Hawaii's elective share rule, which states that you receive a statutory percentage of your spouse's estate if she attempts to write you out of her will, allowing her to leave property to her children instead.
Hawaiian prenups must be in writing and signed by both spouses. In legalese, this is the "statute of frauds." Individuals are only bound by a contract's terms when and if they place their signatures on it. The state also requires that both you and your spouse disclose all property and debts you have at the time you make the agreement, and that you consult with separate attorneys before you sign.
Prenups are legally enforceable contracts, but this doesn't mean you don't have the right to try to convince a court there's a reason it should not honor yours. Hawaiian courts may hold that your prenup is invalid if you can establish you didn't sign it of your own free will, your spouse didn't disclose an accurate picture of her finances, or your agreement is unconscionable. Unconscionable means it's blatantly unfair to one of you. Although some states allow that prenups can become unfair years after their signing because a couple's circumstances change, Hawaii doesn't follow this doctrine. Unconscionability must exist at the time you sign the agreement.
Your prenup isn't enforceable until you actually get married, and it doesn't necessarily have to be forever. You and your spouse can modify its terms or revoke it completely by signing a second document indicating that this is your intent.