I know most highlights can be “refilled” but have no idea how you’d go about doing that.

You can certainly buy refill capsules for Stabilo Boss highlighters - mine tend to get used until the tip wears away. I wonder if you could add extra solvent to dried-out highlighters to extract a bit more ink from them, but I don’t know what the solvent is (isopropyl alcohol perhaps?).

Add in a film can, an old spray bottle, and a bit of surgical tubing, and you have a DIY airbrush:

http://www.instructables.com/id/Airbrush

Some markers can be revived if you inject a bit of water into the dried out part. You buy a syringue from a chemist (very cheap), part fill it with water and then inject a small amount into the ink cartridge. This works really well with coloured markers/kokis (the kind kids use for drawing and school projects). When I was a kid I kept my markers going for ages like this. The intensity of the colour fades each time, but it definitely prolongs the life of the marker. Not sure if this would work well with highlighters, but it’s worth a shot. The drawback is if you inject too much, or if it doesn’t absorb equally, the pen tip can be a bit runny for a while.

http://www.noodlersink.com/

I googled “reusable highlighter” and this came up as a company that sells highlighter ink that you can buy for fountain pens

We put a mix of vinagar and water in the end and let that soak for a day or two - prolongs a bit longer. also just dipping the tip in a bit of water works in a jiffy.

My mother used to revive all of my felt-tip markers by removing the cap on the back end of the marker and dripping a couple of drops of water into the ink reservoir. It usually works, even on dried-up markers.

there is always ink there (even if it doesn’t come out) so here is a neat idea that is really fun. If you have a lot, put them into a container of water and let them sit until all the fluid (or ink) drains out and mixes with the water. you want to be able the see the tint of color. then fill water guns with them. go into a room with black-light and play tag. In the dark it will look like there is paint all over you, but when the real lights turn on you can see nothing because the water concentration is so high.

I did this at the youth group where i work and it turned out great!

How fun!

I guess that you can take a bunch of them and us them to line a little kid’s place mat. I looks really cute and keeps some of those unavoidable bits of cereal, spaghetti, and chips from spreading all over the place (or at least some of it!).