It's easy to appear busy at the workstation staring at the PC everyday (like Alt-Tab'bing to a spreadsheet when your boss passes by), but that's bad practice, especially if you aren't finishing your work in time.
I've seen quite a few people at work not doing a lot of stuff all day long and jumping into their seats once the boss arrives for work in my previous workplace, but I'm not sure if that applies to all companies.
However, if you want a better chance at promotion or a pay rise, here are a few things you could do.
Finish your given work on time, or faster.
Finishing your work on time helps to make it clear that you are interested in working for your company and that you are able to handle the workload easily.
Faster is sometimes better (although not always), but in some companies they might give you fresh work to do without recognizing your ability to work harder than others.
However, that doesn't mean your work should be full of errors. Rely on the saying - "Do it once, do it good!".
Have a break
Taking breaks helps the mind break away from the monotony that is the constant usage of computers and documents. Make it like a reward for working hard for that past hour or two, breathe some fresh air, drink a cup of tea or do whatever you like (just don't breach company rules).
Giving suggestions to improve workflow
If your company allows this, though (and also if you have stayed at the company long enough). Many companies are sufficient enough to handle themselves and may list you as a vigilante that is out to get the company if you're a new hire trying to change things around the company. You might end up getting burnt on a stake.
Make sure you're visible in the workplace
Hey, nobody is going to notice what you're doing for the company if you're going to hide in that pigeon hole everyday without talking to anyone, right?
Get up and talk to people, communicate, try not to make too many enemies and you might be well on your way to be noticed by the supervisor. Remember, you might want to suck up to only the supervisor, but the angry mob forming below you is going to drag you back down. Be nice.