Fax machine II
Change the transmission ID. Most modern fax machines send and identifcation string which is usually the company name. All the outgoing faxes would be shown as coming from, instead of BIG Corp ltd, Madam Zaza's Sex toys Ltd.

Photocopiers and printers
For best effect you will need one of those rubber stamps for [BOLLOCKS] or [FILE UNDER BULLSHIT]. Carefully open a couple of packs of paper and at convenient points stamp the chosen phrase all over one side of the paper. There is a 50% chance that side of the paper wont get looked at before the letter gets posted.

Manuals and handbooks
If you have access to all the manuals for a software package. Why not take a glue stick and stick together all the pages covering installation or some other rarely performed but important section.

Chairs
Many offices have height adjustable chairs. The levers for adjusting the height can be fixed in the up position by attaching cable ties. This is only mildly irritating unless you hide all the wire-cutters/scissors.

Desks
Put some food in the desk of the most annoying person in the office, lock it up and put a GOOD glue in the keyhole. Araldite or other glue without a readily available solvent is best. In a week or so the fish/chicken tikka sandwich should be quite noticeable. Some people dont ever tidy their desks so you could be able to hide it under the mountains of paper and junk in their drawers.

Desks II
Paint the underside with burglar paint, engine oil, mayonaise, the worlds your oyster.

Phones
Re-direct all the incoming calls to a suitable extension and glue up the button needed to transfer calls. Sellotape the handsets down on suitable phones. Picture the busy executive snatching for the receiver and sending the phone crashing to the floor. what better start to his/her monday morning. open the handset and drop something smelly inside, could be about 6 crumbled incense sticks or a peice of crap, depends on the target.

Junkmail
Fill in all those Free-post cards from the magazines like black-box and panel-beaters weekly for free subscriptions. One for every body should be a pleasant way to spend a rainy afternoon. You can get cards for the more obscure ones from your local library.

Plants
Pour a little salt into any plants around your targets desk. He/She will probably get blamed for over watering them.

Floppy Disks
Does your target make regular backups of complete crap. See if you can attach a powerful magnet to the underside of the desk drawer that the disks are kept in. If the drawers are made of wood 5 or 6 drawing pins should give the magnet somthing to stick to. This may never be noticed as backups are rarely used, but they are even more rarely checked once they have been put away for "safe keeping"

Roller Blinds
If your target has a roller blind, staple a suitable poster to the blind and roll it up to conceal the poster. Should be funny when the next sunny day comes along.

Diaries
These respond quite well to the glue stick treatment. Picture again the target on the phone (filled with dog shit) saying "Hold on I'll check my schedule for the week beginning the 7th, Oh shit its all stuck together"

Diaries II
Less irritating but still worthy of consideration is emptying the circles from the hole punch into the diary. When the target flips the diary open the things shower all over the desk.

Diaries III
Use the rubber stamp (See Photocopiers above) to obliterate a week of future appointments.

Diaries III
Hmm must be on a roll here. Use a white wax crayon or candle to rub across the pages at intervals. This makes the pages very difficult to write on.

Computers
These can be great fun if the power plugs are switched around at the wall/floor socket. Suddenly the cleaner unplugs the percolator and the main computer goes down. Also fun if when Employee A goes home and switches off his machine at the same old wall point, Employee B's computer goes off instead.

False ceilings
Does your office have those ceiling tiles that hide an ugly set of girders holding up the next floor above. The ceiling tiles can be spiced up by putting a 1 pound bag of flour on top, just lift a free tile put the bag on the back of the next tile. When the electricians come in to change light bulbs etc. the bags fall out and make a good sized mess. If you are worried about braining some sap wil a bag of flour just pour the flour in a heap on the back of the tile, still makes a mess.

Boring magazines in reception
Glue some Porno-action pictues in at regular intervals to liven then up.