I guess for me main things to have spares... bass drum (pad, or trigger) depends on what you use, one extra pad for the snare and one extra cymbal, and those extra pads don't need to be "extra" expensive, a rubber pad will do as a snare in a hurry and the cymbal can be a pintech single zone that is also very inexpensive. for the Bass drum the Aquarian bass piezo should work very well and it's one of the cheapest out there...
The last things are obviously the spare module if you can have more than one, and one or two cables to replace faulty ones in the even of failure.
I have never had module or cable issues playing live, I have had pads fail...
I was going to do my DIY conversion with internal piezos and a bridge, but I was having problems controlling double triggering, I decided to just buy the $15 DDRUM Red Shot triggers for all 5 toms, and it was plug and play, with the added bonus that If I just wanted to stay acoustic, I can swap heads and back to acoustic. I currently have the Remo Silenstroke and they feel just as good as the Roland, but way cheaper.
Also SOME people don't know that if you use mesh heads on your bass drum, you should not use felt beaters[/u], mine can change to rubber or felt so i just use the rubber side which doesn't chew the mesh head... I've seen that mistake at every music store I have been lately... same goes for sticks, wood tipped are way harder on mesh than plastic... I personally recommend Ahead sticks and not 5B or bigger but something smaller which will help in two areas: it won't damage the set as much, and it will be way easier on you to prevent fatigue..(still do the bananas trick it works). Make a mental (or real) list of your gear so that when you pack to leave you don't forget anything.. I haven't lost gear yet....
Pack a small tool bag (drum key, and other items that you may need.
Like someone was saying here, if you organize your cable bundle, I don't recommend to use electrical tape for the simple reason that it leaves sticky residue and gunks up your cables. I recommend those velcro straps, you can get a pack of 50 or 100 for about 10 dlls... those can be positioned and removed without making a mess. if you want to make it more permanent (but still removable) use zip ties, but cut the tails so that you can be proud of your rack's looks. Also like someone mentioned here, you can buy instrument cables, (long ones for the pads that are further from your module, and custom cut them to fit your rack with just a little bit of slack but not to where they hang too much and take the time to properly solder and insulate your connections, including using heat shrink, remember the better you do it, the longer it will last you).
Finally it may be my OCD speaking but when you pack your Gig bag, Always pack it the same way... if you do that, you will know without looking too much where everything is, it becomes pretty hard to find things inside a black lined bag behind the drums in a really dimly lit stage... and don't forget to carry water maybe 2 bottles, I used to do a 2 hr set and by the time i was done I was dehidrated (beer doesn't help).