Octopods are the most highly developed cephalopods around, and incidentally the most highly developed invertebrate (i.e. spineless) creatures. Usually they do on the seafloor what soldiers do in boot camp, i.e. they crawl around camouflaged, and they lurk in holes.
The camouflage abilities of octopods are nothing short of amazing. Not only can it take any colour or pattern of colours, but it can also change the structure of its skin, by making little warts pop up, for example. It also uses its very stretchy and variable bodily features to take, say, the form of a rocky seafloor. I have also seen a photo where an octopus had camouflaged its body completely as a rock, except for the syphon, which was upright and brightly yellow, so as to resemble a zoophyte.
Having lurked long enough in a hole, the octopus finally attacks his prey, usually by simply flinging its whole eight-tentacled physique over it. If the prey happens to have some kind of shell, it can either rip it apart by simply applying enough tentacles on either side and pulling or it can pry it open with his beak, which is located at the centre of his body, inmidst of the tentacles.
The modes of locomotion of an octopus are a great many. It can use its syphon as a little pumpjet to propel itself forward, head first; it then gathers its arms together behind the head and takes a streamlined shape. On the seafloor, many different ways of moving across the ground, using some or all of the tentacles, are employed, some slow (such as a sinister crawl with the tentacles curled up nearly completely), some fast (the beast stretched out lengthwise, and pulling on rocks tentacle-over-tentacle).
It is possible to teach an octopus many things, including distinguishing shapes and colours. Less scientifically useful, but more fun to watch is teaching an octopus how to pull corks out of bottles. Marine biology students with too much spare time tend to do that. It's arguably cooler than using a corkscrew.
Worth noting: On land, an octopus is about as helpless as a wet sock, but chiefly because of the relative dryness; amazing as it sounds, the boneless little beast can move outside of the water, but apparently only over comparably slippery surfaces. It looks like a huge puddle of strangely animated barf and pulls itself forward with an oozing motion. For example, it can crawl off a fisherboat's deck back into the water under its own power. I use to wonder why cephalopods have not made it on land.
Some octopuses have a garden.