I am currently working on a project which involves the pictured piece of driftwood. So far I have been cleaning the wood manually with sand paper, but this is quite slow and exhausting. What type of (preferably consumer grade) tools can I use to speed up the process?
OK so you had loads of good advice, but I've done a very similar clean up on a branch, and I have the cheapest option to recommend: I've debarked my branch with a nail.
The flat head of the nail can be used to 'crowbar' some pieces of bark, by leveraging the length of the nail to apply pressure. You can also use the side of the head to push off bark from knots and the side of the branch.
To finish on my projects, I had to eventually use a metal file mill + sand paper. I intend to do more projects like this so I'll almost assuredly get some of the tools listed in here, tho.
What kind of cleaning are you trying to accomplish?
If you just want to remove dirt and debris, use a nylon bristle brush. If you want to remove loose splinters from the surface, use a brass wire brush. If you want to scrub the surface down to accept stain/finish without completely smoothing out the original texture, use a steel wire brush.
Oh, yeah for removing the loose bark you probably want to do the steel wire first, then the nylon to remove any dust or small flakes. You could also do steel wool, but depending on how rough the surface is it might shred and leave little bits behind.
For an irregular surface like this you might also consider spray polyurethane for finishing, it will be easier to apply and the surface doesn't have to be perfectly smooth.
I use the bristle brushes but in drill bit form. I've also got a bunch of Roloc^TM sanding bits for my m12 polisher. I've mostly used it on cherry and walnut offcuts from the garden. Careful with driftwood as it's often very soft / light weight wood that power tools might rip through.