For the example, I made a circlet of blasting that can be activated by the user to fire a mage bolt.
The first issue has to do with trying to use equipped items. In the screenshot below, the character has the circlet equipped on his head slot. But when the player clicks on the item, there is no option to use it, only the box about not being able to equip/unequip during combat. Had it been a weapon or shield item, then clicking would only go to the inventory screen to unequip or swap it, but again no option to use the item.
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Now, it is possible to use the item by keeping it in the party inventory. This means anyone can click on the circlet on their turn, and they will get the option to use the item. The problem here is that sometimes the character can use the item this way, and their turn does not end. Thus they can take another action, or even use the item a second time, before the character's turn ends. In the screenshot below, Greenwood has used the circlet of blasting twice in one turn.
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A third issue has to do with charges that can be assigned to an item's blueprint in the toolset. As shown below, the circlet of blasting has 5 charges. However, working from the inventory, the party was able to keep using the circlet in battle. Everyone on their turn could activate the mage bolt, some even twice in a turn, as noted above. I would say they easily got 10 uses out of the item, despite the property of 5 charges. It seems the only option in the toolset is to destroy an item after one use, like potions. Below is a screenshot of the circlet's blueprint in the toolset.
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At this time, there are some workarounds to make useable magic items. They cannot be equipped (or if they have equipped properties such as resistances, they must be unequipped and back in the party inventory in order to be used). And it would seem best to make an item one use only, being destroyed after activation, if you don't want it to be used indefinitely. It would certainly cause players to consider the right time to expend such a limited use item.
Obviously, this is not the ultimate intention. Wands, rings, staves, etc. ought to have charges to allow multiple uses before disappearing. And it would be nice to make necklaces, or really any kind of item that can be equipped by a character, and give it a useable ability as well. Hopefully this will help illustrate what needs to be functioning so we can have a variety of creative itemization.