Without killing yourself or others?
The best way to open a brown hairy coconut, not a fresh green one (assuming you are right handed, I imagine that sinister people can reverse the instructions, but I've not tried it).
First image the coconut is a head, everything is related to this, so the visualization is important. The three holes are the eyes and mouth of the face. They naturally form an eyes/mouth thing. you'll see that it only really looks like a eyes/mouth one way up. stare into its eyes (this bit is not so important, but I like to do it).
Now hold it in your left palm, under its chin/where the neck would be, with the 3 holes in the coconut arranged like the eyes and mouth of a face looking out to the right, mouth below the eyes, with the hair pointing up. There will be a slight 'seam' running between the eyes and back over the top of the head.
Then take a large, heavy knife and using the FLAT side of the knife (not the sharp edge), a meat cleaver works best, strike the coconut hard across the coconut's 'forehead', where its hair line would be, aiming to impact exactly on the 'seam'. You might need a few goes to get the crack right open, but sometimes it can be done in a single strike.
Get this right and it will crack in a straight line from ear to ear along the hair line. Works a treat. And usually you can use the shell for something as you get a clean break.
Once you have the first crack you can either prise apart or rotate the coconut in your hand and hit it a couple more times on the uncracked parts, following the same line as the first crack, to completely separate the two pieces.
Alternatively you can stick it into the oven for 15 mins, @400F, after which the shell should have started to crack. Then you can use the tool of choice to finish the cracking, rolling pin, hammer, wrench, flat of knife another coconut, whatever. But where is the fun in that?
The easiest way is to use oven. I find that 190C/375F is convenient.
Before placing it in the oven you should drain the water from the coconut. Drilling the 2 of the 3 holes with a hammer + screwdriver works fine.
After removing from the oven after about 15 minutes use a hammer to easily crack open the coconut. If it's not easily cracked, let rest for a few more minutes.
The brute force way is with a machete :) However, you can also do it with a small handsaw (which is useful if you want to use the shell for something - they make cool hiding places for fishes in an aquarium). Otherwise, you can split it with a wide chisel and a hammer.
If you're just looking to get the milk out, poke two of the "eyes" and drain. Don't laugh, but find a concrete floor and quickly bang the coconut and it will split into fairly even pieces. Be careful because the edges can get sharp....
Along the circumference of the coconut, draw line with water. This creates a fissure line. Then hit hard on this line with a small rock or equivalent & Voila! Dont believe me? Try it!
Here's a video of how to do it:
When I was a child, my father would bring coconuts home. He would puncture the eyes and drain the juice, then my siblings and I would run up to a second-story window and drop the coconut onto the asphalt. Worked pretty well.
Since you didn't say you want to retain the coconut water, just use a big ol' axe!