Hopefully, you will be using it for more than just chasing. It's a fair sized camera. (Same as the XL1 and XL1s). Here's a short list in the order in which I would get them.
1. UV Filter
2. Circular Polarizing Filter
3. Extra Batteries
4. A different Mic. The Canon Mic is not a true "Shotgun" Mic. I use an Audio Technica 835b. You will also need an XLR to Mini Jack adapter for this.
5. Canon MA-200 Shoulder rest and Audio Adapter - Very versatile piece to have.
6. Battery Powered Camera Light - The XL2 is somewhat better in lowlight than it's predecessors, but full dark you will need a light.
7. Good Tripod - This camera WILL get heavy
8. Canon C910 Dual Battery Charger and Adapter - allows you to use two batteries on the camera and increases your shooting time.
9. Rain Cover - This should be self explanatory. Trash bags, though will keep you from ruining your camera, don't work very well and are noisy.
10. The Canon 3X Wide Angle Lens - provided you can afford it. If not that, then the Century Optics WA Adapter Lens. This is a MUST for close in shooting.
I could go on and on, but those are the top 10 items I would start looking at.
Of course it would help to know what the primary use for the camera will be. Just off the cuff shooting of birthdays and around the house kind of stuff doesn't sound right as the XL2 is a semi-pro camera and has some very nice potential to it. If the primary use is going to be chasing, I would seriously reconsider the camera choice. There are other 3 CCD cameras that will serve you better in the confined space of a car and are way more portable than the XL2.