Another good idea for stacking item properties is a weapon crystal or armor crystal (especially if you're a warforged monk).

Stacking weapon crystals is an extremely cheap way of increasing your damage output and protecting your weapon from sundering, especially compared to adding the exponentially increasing +1d6 damage boosters. You're getting +1d6 acid/cold/electricity/fire/sonic regardless, but one is a scaling cost while the other is a flat (and relatively small) cost, even with the +50% markup.

As far as warforged monks go, you're adding the armor crystal onto your warforged plating, which IS you. It's your body. So getting that +10 hardness affects any damage you take that doesn't explicitly override it, which is very nice. Of course, this is one case wherein enhancement bonuses to AC are great as well, since those explicitly increase HP and hardness ratings, and since your armor IS you...