My other advice is to key it to the PC it's planned for. Find ways to make its abilities both (a) resemble those of other items of its type - in the case of tattoos, either "when you spend an action point, X happens" or "static bonus Y depends on the number of surges you have spent today" - and (b) both interest, and challenge, the player it's given to.
For instance, the rogue in my game has a really strong paranoid streak. Unless it really cuts into optimal play, he'll always choose the safe tactic or safe option. So when I was tossing him his second +1 Dagger (the first having been a Parrying Dagger with a defensive bias), I altered the Critical line. Instead of +1d6 for a normal Magic Dagger +1, his reads "Critical: You may roll a death save. If you do so, whether or not it succeeds, add +2d8 damage."
I wouldn't give this item to just any character; if they didn't object to death saves so much, it'd be just an intellectual tradeoff. But for Crumpleface Jack, it's a dare. It's an RP hook masquerading as a Critical property. (I've even gone more explicitly RP than this. Our cleric, who is technically a slave and still sort of considers himself one, has a slave collar as his holy symbol, whose Crit listing is "+1d10 damage and you feel worthless for being without a master.")
So give them things which will make the specific PCs you're aiming them at have to make nontrivial choices, and choices which make them get further into character. At that point, precise setting of levels and precise power of items becomes less important, simply because that's no longer the whole focus.
If you want a hand, post a quick background/build summary of the PC you're planning it for, and maybe we can come up with some suggestions.