Definitely want a notebook and something to keep it in so it stays dry, and don't forget a pen
and a pencil. I use one of those crappy little short pencils you get in places like Ikea and in the booths at polling stations, and I've sharpened it at both ends on the offchance the lead in it breaks, other one is a short black felt tip type pen (biros will often stop working in cold and damp, so avoid those, but at a push, you can get a biro going in the cold by scribbling on the rubber heel of your boot). Make some notes at the morning briefing, as it's often difficult to remember all the details later on.
I'd recommend having a plan view map of the site, with the scale and elevation marked so you will know approximate distances and slopes, especially if you don't know the place. You can 'google earth it', which will tell you elevations above sea level and distances, then use that info to draw a simplified version of the layout onto paper, or thin material if you want it to be easily foldable and more durable, you can 'laminate' paper with clingfilm and sellotape if you want it to be waterproof but still foldable, and doing that will also let you mark it with a felt tipped pen which can later be rubbed off. Your map doesn't have to be a work of art, keep it simple and you can use it to point stuff out to others. If it is a large site, then remember to take a compass, or your map will be useless to you unless you familiarise yourself with a few easily recognisable locations, after all, a recce is what any half decent soldier will do. At a push, it's often the case that moss will grow mostly on the northern side of trees and outbuildings, so you can (sometimes) use that to orient yourself if you don't have a compass.
If you and your buddy don't have radios, then you should get some, ideally what you want is some PMR 446 radios. Don't have to spend a fortune, see here for some cheap (13 quid for a pair) but usable ones:
http://www.ebay.co.uk/itm/111331756228?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Don't forget some spare batteries for them (AAA), and check that they work, anyone who's ever read about Bravo Two Zero will know how tits up things can go if you don't have a working radio, and there is a very good chance that others at the milsim will be using PMRs, so make a note of frequencies and callsigns they are using in that notebook. Nothing worse than having a radio and not having the correct freqs, again, see Bravo Two Zero for some world-class stupidity on that score.
Some sort of watch is handy, don't rely on the clock on your mobile phone, that's too much arsing about in a firefight, use an actual wristwatch, with a second hand, and make sure it is set to GMT exactly to the second, because everyone else who is taking it seriously will have done that, so that they can coordinate operations, and you don't wanna look like the 'FNG'. If you wanna go all 'authentic' here's where you can get a US army watch cheap:
http://www.sofmilitary.co.uk/mwc-us-military-watch.-product,12312
If you wanna go all 'Nam', here's where you can get a period 1969 US army watch:
http://www.sofmilitary.co.uk/us-army-vietnam-watch-product,14376
Obviously you need some cooking gear for making a brew and stuff, take some (waterproof) matches. You can simply rub ordinary matches on a candle to make them waterproof, although make sure they are not safety matches, you want the ones you can strike on anything. I'd also recommend a Zippo (or a cheap two quid knockoff copy of one will do, since a genuine Zippo is about 25 quid). A Zippo will light (and stay lit) even in very strong wind and rain, which is what you want when trying to light an army stove. Fill the thing up right before you set off and it will last all weekend, and make sure the flint is good. Don't put a just-filled Zippo in your pocket close to your skin, leave it a while to let some of the petrol evaporate or the petrol will irritate your skin when it comes into contact with it when in your pocket.
If you don't have cooking gear, look on ebay for 'hexamine stove' (beloved of armies and survivalist nutters all around the world), you should find one for about three quid, although you might want to get more fuels bricks for it than it comes with. They are small and don't weigh much at all. Don't forget a mess tin and some utensils (wash them out if they are new). Note that hexamine bricks are a skin irritant, so don't handle them for prolonged periods, preferably use gloves, and don't use a hexamine stove in an enclosed space, the bricks emit some toxic fumes when burning, which is fine out in the open, but not good in an enclosed space. Don't let a hexamine brick touch your food either, they are not finger-lickin' good. When it comes to food to take, things such as oatmeal blocks, instant packet soup, crackers and that kind of thing are good, being light and providing a decent amount of calories (reckon on burning about 2,000 calories a day and you'll be able to work out how much stuff to take). A stiff tea with craploads of sugar in it will give you plenty of energy fast, and some teabags, sugar packets and powdered milk do not weigh much or take up much space. Don't forget a water bottle of course, it's hard to make a brew without water!
Take some gaffer/duct tape and a folding knife. You can fix almost anything with those two things, so you should always have that in your webbing.
