This is a catch-all category for anything that doesn’t fall into the other groups. Anyone can use a wondrous item
Basically a Wondrous Item is any magical item that isn't one of the following: Magic Armor, Magic Weapons, Potions, Rings, Rods, Scrolls, Staves, Runes, Wands
They can be small one-use items (ex. Feather Token), or slotted (ie. something that occupies an equipment slot, head, belt, back etc) adventuring gear (cloak of resistance +1), or really anything else (that isn't in one of the above categories). As for stating, they can really be anything you want, although there are some guidelines for power balance (like not recreating existing items in different slots).
I'm assuming you've read this page (or something similar) on magic item creating, since you are referencing some of it in your question. The general way to create magic items is to find a spell that does the effect you are looking for, and base it off that. If you want to make a helmet with an activatable ability, you are essentially in this line:
Spell level × caster level (that the spell is being used at) × 1,800 gp
Then, since you want it 3 times per day, you divide the cost: Cost / (5/3)
Of note is that creating balanced custom magic items is somewhat of an art, and acceptable items vary greatly between GMs, and every custom item needs GM approval.
An Example of something that might be what you want:
Crown of Detect Magic
3 times per day, cast Detect Magic for up to 10 minutes at a time. Note that this is a head slot magic item, NOT magic armor, so it doesn't confer any AC bonus on its own.
Spell Level = 0, so 0.5 for pricing concerns
Caster Level = should be 10, since it's for 10 minutes, but really Detect Magic is meant to be used frequently for short (1-3 rounds) periods of time and requires continual concentration, so this will require some judgement on our part. Lets say 3 for now.
So far we have: (0.5 x 3 x 1800) / (5/3) = 1600 (rounded). I'd say this is a little high, so lets take CL down to 2, so cost = 1100.
Compare to similar items: Scarab, Golembane has a detection aspect and costs 2500, but it also confers combat bonuses.
Consider the cost to use with Permanency: It states that making this permanent (which would not take up a slot) is 500xp. I would ignore this, since it isn't a large amount, and the benefit of gaining an ability slotless isn't applicable here.
So, I'd leave the cost as is, but change the ability to be:
When activated (3 times/day), the user can cast detect magic at will as a free action for 10 minutes. The user must still concentrate to gain benefit from the spell.
You could also go a different way and have the ability be as if you were concentrating constantly without any effort on the users part, but that would increase the price.