Special Abilities
Source GM Core pg. 134
When deciding what special abilities are appropriate for what level, it's best to look for similar spells to gauge the effect. For most consumables, the effect should be less powerful than the highest rank spell a spellcaster of the item's level could cast. Spell gems are about the most efficient you can get—they're the same level the spellcaster would be—but they require a spellcaster that has the spell on their list, and take the same actions as casting the spell normally.
The most straightforward choice is a once-per-day ability. For this, the item's level should be at least 2 levels higher than the minimum level a spellcaster could first cast that spell. For example, if your ability is about as powerful as a 3rd-rank spell cast once per day (perhaps haste), then it should be at least a 7th-level item. A basic spell chip is a good example. However, a spell chip is flexible and can contain the most effective possible choice for its spell rank (such as long-lasting spells where once a day is effectively permanent), so a specific item that doesn't grant such a spell could have additional powers or bonuses at the same price as a spell chip.
If the item can be activated multiple times per day, it should be at least 4 levels higher instead—9th level in our example. Frequency could range from twice per day to once per hour and anything in between. Choose whatever makes sense to allow the characters to use the item more frequently without being effectively constant or unlimited. The appropriate frequency, or whether it's ever okay to have unlimited activations, varies wildly based on the spell. Unlimited castings of a cantrip is fine, but an effect akin to a non-cantrip spell is rarely a good idea. Only attempt to build such an item when you're certain of the consequences.
Items that can be activated less often than once per day don't appear too often, and they usually fit best with abilities that make sense outside of encounters. It's still best to stick to the guidelines for once-per-day abilities, but these items tend to have more properties—and often strange ones.
The most straightforward choice is a once-per-day ability. For this, the item's level should be at least 2 levels higher than the minimum level a spellcaster could first cast that spell. For example, if your ability is about as powerful as a 3rd-rank spell cast once per day (perhaps haste), then it should be at least a 7th-level item. A basic spell chip is a good example. However, a spell chip is flexible and can contain the most effective possible choice for its spell rank (such as long-lasting spells where once a day is effectively permanent), so a specific item that doesn't grant such a spell could have additional powers or bonuses at the same price as a spell chip.
If the item can be activated multiple times per day, it should be at least 4 levels higher instead—9th level in our example. Frequency could range from twice per day to once per hour and anything in between. Choose whatever makes sense to allow the characters to use the item more frequently without being effectively constant or unlimited. The appropriate frequency, or whether it's ever okay to have unlimited activations, varies wildly based on the spell. Unlimited castings of a cantrip is fine, but an effect akin to a non-cantrip spell is rarely a good idea. Only attempt to build such an item when you're certain of the consequences.
Items that can be activated less often than once per day don't appear too often, and they usually fit best with abilities that make sense outside of encounters. It's still best to stick to the guidelines for once-per-day abilities, but these items tend to have more properties—and often strange ones.
Constant Abilities
Source GM Core pg. 135
If you want an effect to be constant, set the level and Price accordingly. For instance, let's say your group is 9th level and you want to give them an item themed around allowing the entire party to fly. The 3rd-level augmentation ultralight wings provide a fly Speed for one character, and a 5th-rank casting of the wild bond focus spell gives most parties a fly Speed, lasts a minute, and can be cast multiple times per day using the Refocus activity, so it can cover flight by 9th level. To keep it simpler, you decide to create a 9th-level magic item that creates a swell of primal energy causing allies around you to grow wings and gain a fly Speed for 24 hours. Remember, some effects were never meant to be constant and could warp your game.
Activation Actions
Source GM Core pg. 135
Watch out when picking the number of actions an activation takes! A 1-action activation that casts a spell with a 2-action casting time is drastically more powerful in an encounter than an item with a 2-action activation would be. An item like that is typically much higher level, and it works best with “helper” spells or ones with limited utility rather than offensive spells. The safest bet is to use the same number of actions the spell normally takes to cast.
Scaling out of Usefulness
Source GM Core pg. 135
Some spells aren't appealing if their rank is too low. For instance, an item that casts 1st-rank overheat three times per day might be 5th or 6th level. The problem is that spell scaling has the biggest impact at low levels, so the spell isn't effective compared to other actions a character could take. Err on the side of fewer, more impressive activations.