If you dig paint as much as I do, you probably realize how easy it is to waste lots of it. I'm slowly getting better at judging how much paint to use when I'm mixing colours, but it's still a tricky process. More frequently than I'd like, I end up with more than I need.
Occasionally I save it in a small container, and then forget about it - only to open it months later and find the paint is completely dry.
What I've ended up doing over the years is putting that leftover paint to more productive use in a couple of ongoing projects. Smearing the stuff onto a glass palette can come in handy - once you've got a nice thick layer of paint built up, you can peel it off and cut it into pieces to use in new paintings (trivia note: these are referred to as "paint skins"). You can do all sorts of cool stuff with them - building impressionistic colour, creating texture, making mosaics, etc. etc. Just google "acrylic paint skins" and you'll find lots of articles and tutorials like this one.
I've also got a couple of long-term projects in the works using the leftover bits. Both of them started with me just innocently wiping my brush or knife on the nearest convenient surface. They're still transitional pieces, and probably will be for a long time to come, but eventually they'll be full-fledged works of art on their own.
The first is this former coffee mug:
A souvenir of my first advertising job, I still use it to clean my brushes. It's gradually getting so coated in paint that the bigger brushes will barely fit into it. I also do a lot of colour experimentation on the surface of this thing. No idea where this will take me, but for now I just want to make it a lot wider (possibly even spherical) with a semi-lattice effect.
And the other piece is this old beach towel that I use as a drop cloth:
It's still quite flexible now, but once it's too stiff to roll up, I'll mount it to some kind of support and make an actual painting out of it. I can almost see the design in my head, but not quite. My subconscious needs more time, I guess. I suspect there'll be geometric shapes laid out in gold leaf on top of the final painted surface.
Kinda makes me wonder what else I could do with leftover paint.