Any magic item, except consumables, really.
The potential exists for abuse with regards to items without scaling benefits, but then I just have a rule requiring that such items are at least within a certain range of your level.
The only other thing to do is work out how crit bonuses on weapons work, and that's easy; they overlap. Use whichever from your weapons or innate bonuses is better. This keeps things like Vicious Weapon useful.
I should also mention that I've given every class Implement Proficiency with Ki Focuses and made all weapon enchantments Ki Focus implements. That further adds to the "Part of the character, not his stuff, unless you want it to be" setup I'm going for.
EDIT: Also, I figured it went without saying, but enchantments still take up the appropriate slot. Those Slippers of Spider Climbing may represent everything from a natural ability at climbing to a cantrip to actual magic slippers, but in each case it still takes up the foot slot.