I just bought a 2011 Trek 520, brand new from my LBS. It had the Interchange Rack already attached. I want a trunk bag. Can anyone recommend the Interchange bags, or should I consider another brand? Also, what other specific (brand/model) accessories would be helpful for day/night commuting? I want fenders, for sure.

Lights, of course, for night commuting.

Planet bike fenders, only 20ish for the pair. personally I have a led flashlight and 2 hose clamps for my headlight and works great. Flashlight is from Home Depot and has flashing capability too, cost 5 bucks. I'm a DIY cheap bastard so bear with me....kick stand, highly recommended. Greenfield has black one I like looks way better than the plain al one they have. It was like 13 bucks at my lbs. I splurged and got panniers, axiom, 75 for the set but there are tons of DIY ideas if you're into that. If not break down and get them, whatever brand you decide on. Dont have a trunk though so no comment. Oh, and a comfy saddle. I use a brooks but is susceptible to rain...the botranger saddle that came with my 520 made my toes go numb in 15 minutes. Spd pedals with a platform on one side, highly recommend. I don't clip in unless I'm going to work or a tour and I use my 520 for everything. A good lock, aluminum water bottles is probably my most stupid priced accessory but I love them and use them for camping(I can boil water in them) they're at rei and fit into the cage, brand is slipping my mind something green I think. Frame saver, get it done. Still run the botranger race lite hard case tires that were original on the bike, no issues with them at all...tool kit, that's all I got. Oh yeah and a cheap cateye computer, love logging miles.

Oh forgot to go into tool kit; park multi tool kit, love it, crank bros has a very nice one as well. Tire levers, just the cheap plastic ones. Park mini pump; don't need frame pump for commuting got the floor pump at home(get one if you don't already have one), spare tube, Rema patch kit, and my gerber. I do about 85 percent of my maintenance from that pocket of tools, a can of pledge, a spoke wrench, degreaser, rubbing alcohol for cleaning the rims and chain lube. Oh, and my work stand. Got one for 65 on amazon. It's a no name but it works great and I don't know how i lived without it.

Since you already have the interchange rack, you might as well go with the interchange bag, as it is designed to fit perfectly on the rack, and has a special mounting system, so easy on and off with the bag. https://www.youtube.com/watch?v=glKIRjJbxNk

And more lights for when you realize you need more light to be safe and discover the benefits of using a light on your helmet. And some reflective clothing.

Is that the dynohub version?