Some sort of cooling fan. No motorcycle, whether air- or liquid-cooled, should be left to sit and idle without the benefit of cooling air blowing across it to prevent overheating. Overheated engines don't respond well to subtle adjustments, and if it gets really hot there's a chance you may damage something. An old window or box fan will work just fine, and new ones can usually be bought for under 20 bucks at the nearest Home Depot.
An auxiliary fuel tank will come in handy if you need to remove the bike's tank to access the carbs. It won't be required in all circumstances, but if the tank has to come off, the backup container is a lot easier to use than an old coffee can with a couple of holes punched through it. Motion Pro makes a nice one that's reasonably priced.
A dedicated carb-adjustment tool. Again, this isn't a strict requirement, so check the manual before you spend your dough, but in some cases one of these tools, which usually contains a screwdriver tip and lock-nut socket combined into one unit, will make life a whole lot easier.
Vacuum gauges, technically called manometers, are the one must-have item-without them you're dead in the water. Vacuum gauges come in a variety of styles and shapes. Right off the bat I'm going to recommend you pass on the analog-style multiple-gauge setups as well as the type that uses a floating steel ball. Both of those need to be calibrated at each use and require a damping adjustment if you want to get an accurate reading. Mostly they are more trouble than they're worth, especially for the casual user.
Tossing a spare in the saddlebag, tool kit or maybe a stash pouch is always popular, but make sure the spare is accessible without unlocking anything!
I've seen quite a few guys wire or tie-wrap a spare to some out-of-the-way place on the frame. Make sure the hiding spot is well protected and out of sight, and don't forget where you hid it.
Inside the headlight is a great spot-warm, dry and protected, and you can usually pop the headlight off without too much trouble.
Inside the taillight-ditto. When I was a dealer we always stashed a spare key in the taillight just in case. You'll need a screwdriver to remove the lens, but they're usually easy enough to come by, and in a worst-case scenario a rock makes a passable substitute.
Inside the handlebar may work but it can be tricky. In the first place, modern handlebars aren't always hollow, and in the second, you'll normally need tools to remove the throttle or a razor blade to remove the left-side grip and retrieve your hidden key. But either method is preferable to the alternative. If you do hide the key in the bars make sure it can't slip all the way to the center. Foam wrapped around the key will usually do the trick.
A hide-a-key box can be bolted, tie-wrapped or even duct-taped to the inside of the fender or frame or even to a convenient engine bolt. Tape the box shut and you should be all set.