The most important thing you always want to remember about placing your humidifier, is to put it somewhere where there won't be water damage. Figuring this out can be a lot more tricky than it sounds because sometimes it takes some time before water damage becomes apparent and...by then, it may be too late to stop or undo the damage. 

So... a plant saucer or water-proof placemat is something you'll want to think about keeping underneath your humidifier at all times, no matter where you choose to put it. 

Some plants enjoy literally having their leaves moist all day. Things like carnivorous plants (like pitcher plants or venus fly traps), elephant ears, and banana plants enjoy a high level of humidity. These are often fast growing plants that produce by pup(ping), have a lot of leaves, and may not have any wood or trunks at all. 

Have you tried growing elephant ears? If you're not sandwiching your humidfier among an array of plants but are a collector of 24 inch planters, you might want to try sitting your humidifier on top of the soil.

Plants that need a lot of water to thrive where there's space for your humidifier, might just be the best place for it. The leaves will be kept constantly moist which plants that are native to bogs and swamps need to prevent leaf-crisping and to stay alive indoors. 

But seriously? What's the highest place in your urban jungle set-up that you can put your humidifier? Do you have a shelf up high? Can you hang it from the ceiling?

The higher up your humidifier is, the more space it has to diffuse throughout the room. This helps prevent water from condensing on whatever surfaces might be below your humidifier. Having your humidifier up this high also gives you the option to turn the humidifier onto a higher setting to keep your urban jungle's environment extra moist. This is something to think about because even at 40% humidity, that is still a much lower humidity than many jungle plants are used to. If you can get that humidity up even higher, then that would be ideal for the health and happiness of your house plants. 

Wood floors? Stains and rot. Carpets? Mold and continually damp carpets. You don't want either of these problems so avoid them and just don't keep your humidifier on the floor UNLESS it is made specifically for being on the floor. 

There are humidifiers that are somewhat reminiscent of elephants in the way that they reach upwards and spray their mist into the sky, allowing it to disperse without puddling or condensing even while sitting on the floor. Humidifiers are becoming better and better everyday. 

But unless you have a humdifier especially designed for floor-use, I'm going to have to say, do not put your humidifier on the floor. You'll be very sorry in about a month.