First off they're usually good for water marbelling as they spread well in the water (most of the time, but this isn't guaranteed)
Secondly you can layer them. Experiment layering over white, black, and any other colours you think may work. Glitters tend to work well over black, pastels over white, or simply an opaque version of the thin polish you're trying to use up.
So try layering your thin polishes over more opaque bases to see what you can create. Sometimes even strange colour combos can look incredible so start experimenting!
Finally thin polishes are perfect for sponging. Thicker more opaque formulas will put too much polish on your nail when you sponge with them, making it extremely easy to ruin your work. However thin polishes can be built up slowly, and because they're thin you'll get a fabulous gradient effect with them without much effort. I used Fantasy Fire in this gradient video below, notice how I slowly build up the colour with it. If it was a thicker polish.... well....one touch of the sponge and I'd have a dark splodge on my nail :/
So if none of those options appeal to you let me explain how to thicken your nail polish.
You simply need to use air, but it's important to do it carefully.
Open up the polish bottle and take out the brush. Wrap it in cling film/shrink wrap to help stop it going too stiff and put it somewhere safe. You might want to clean it up with tissues and polish remover as it will be a while until you use it again.
Place the open bottle in a glass beaker to stop anyone knocking it over accidently and put it on a shelf somewhere safe, where no pets or kids can get at it.
IMPORTANT - put it in a room that you don't spend a lot of time in. The solvents in the polish will be released in to the air around you and, if you've ever spent a long time painting your nails you'll know this, it can cause headaches and really isn't very healthy. So you don't want to dry your open polish bottle in your bedroom or living room, or any room that you spend a long amount of time in.
I put the bottle in my kitchen/diner. The back door was being opened regularly which provided ventilation and the room was big enough so that you couldn't smell the polish.
Leave it there for a day and check it. Depending on the heat of the room and the climate you live in it could take a few days or even a couple of weeks for your polish to thicken.
Make sure you check it regularly as this isn't an exact science. Be aware that the polish could go gloopy or lumpy, get too thick or just be a bit weird.
Which is why I prefer to layer my thin polishes, marble with them or use them for sponging.
But heck, you never know, you might just end up thickening your polish enough to make it way more wearable!