Evaporative, cool mist, dry mist, ultrasonic? Where should I start? The youngest needs something cheap for a couple of years in a different area of the country.
We use several of those cool mist humidifiers around our house in the winter when the air gets really dry. Had them for years. You do have to babysit (watch/refill) them and protect the floor around them. They can shed a bit of white/powdery residue–minerals from the water if you use tap water. (Using distilled water would prevent that–but I use gallons a day, so using distilled water is impractical/ too expensive for me). The floor can get wet if you turn it up too high. But they are inexpensive and portable, don’t get moldy or smelly, and there is no danger from overheating if the water runs out and it is left on. And they use less energy than the heated ones.
Also seems to help to change filters regularly and add anti microbial treatment to the water as you go.
Our cool mist units (Sunbeam SUL495 Ultrasonic) have no filters. No need for antimicrobial. Sometimes I clean off the lime buildup before I put them away for the summer.
I picked up a small cool mist one at Target a couple of months ago on clearance. No filters to clean but by using distilled water (you go though a gallon a day if you run it 24/7) you don’t have the white dust that you can get from using tap water or any build of up gunk in the machine. It’s a Crane ultrasonic personal cool mist humidifier. If this is for use in an apartment, something I found handy a long time ago was to run a second shower curtain tension rod down the middle of the tub (above the shower head). You can use it to drip dry clothes with no mess. We also get a hand towel dripping wet and hang it to add moisture to the air. It drips into the tub while adding moisture.