
Of course, don't forget the synthetic oil, extra plugs and filters, whatever cables/cords you need, transfer switch, gas cans, and the all important carbon-monoxide (CO) detector.
But once you've gotten the basics covered - then what? Might I suggest getting a "Kill a Watt" energy meter. One use for it in this case is to measure the current required by your existing electrical stuff to get an idea of how much you can run at once. Also, generators should be "load balanced." It's important to realize that even though you may have a 5000Watt gen., in most cases, it is really 2 seperate circuits that can each power 2500watts. It's also been reported that severely imbalanced loads can shorten the life of the alternator.
It's also vital to know that electricity put out you're generator is up to snuff. Most of the consumer level generators have no meters. You're lucky to even get a gas guage on one! A 'KAW' can measure the voltage and frequency of your generator output along with a few other things. Of course there's more to power quality than just that, but most people don't have an oscilloscope laying around and this is better than nothing.
The best price I have found on a Kill a watt is $18.99 at buy.com
While there get your generator something to read. Item# 33769015 - Small Engine Care & Repair: A Step-By-Step Guide to Maintaining Your Small Engine for $9.76. I hear repair services can be difficult to obtain right when you need them the most. The total cost of the 2 things will get you free shipping to boot

May the Force(power) be with you.