Things I have done when bored at work, though I whole heartedly do not recommend you do the same, unless you actually do want to get sacked and possibly taken to court..
1. Go to toilet and sleep in one of the stalls for 20mins.
2. Create a makeshift ball out of celotape - play with as you see fit.
3. Start a 'playfight' with a colleague and proceed to fall through a plaster board wall, leaving a nice 6ftx6ft gap where said wall used to be.
4. Jump on a fork trolley and use it as a makeshift scooter. (Advice: do not crash into shelving support strut, the shelves will fall down and everything on them will break)
5. If you are a smoker, have a cig out back of shop. Return 15mins later to find said cig has ignited just about everything in sight. Phone fire brigade immediately.
6. Make odd noises over tannoy system. For added fun, time it so the manager walks in on you just as you crack up laughing.
7. If you have a TV section, which all play from the same DVD/Video player.. add a video from your own 'collection' to the tape silo. Prepare to handle many customer complaints.
8. Silly gags - e.g. celotape on the earpiece of phone handset. Bonus points for manager using it infront of customers.
9. Bubble wrap a colleague, shove them in a large box. When delivery drivers arrive to collect, ask them to play along and 'deliver' this box somewhere. (Normally a short stint around the block and back to the shop will suffice.)
10. Fill out customer returns with fake details - the obvious names like I. P. Freely apply.
11. Play comp games on work machines. Bonus points for multiplayer games with colleages. Super bonus points for shouting "owned!" unexpectedly in your quiet office, when you achieve a satisfying kill.
12. Have a 'team meeting'. In the pub.
and the list goes on, but my manager approaches..