I use a small Timbuk 2 seat bag. What I've found is that any seat bag small enough to not annoy your legs is going to be too small to carry enough stuff without resorting to your pockets on longer rides.
There are a couple of good points to the Timbuk 2. It has solid attachment and is easy to install and remove. The fabric is bulletproof (I think literally). The zipper is high quality and situated such that call the crap doesn't fall out when you open same. It is stylish grey with a yellow circle on the back.
There are also some bad points. It has no reflective material anywhere and has no tab on which you could easily mount a flasher light. One or the other of these should be mandatory.
Usual contents of mine: patch kit, Topeak Alien tool, Pedro's yellow tire lever, new CO2 thing that's just the chuck head but only works with screw on cylinders, 2 16oz screw on cylinders, SRAM chain link thing. This is all I carry on most rides. On longer rides I pack a tube, $$ and whatever I'm hungry for (ribs, turkey leg) in my jersey pocket.
Sometimes you dork out, as I did at Poolesville when I forgot to take my bag off. Sometimes you dope out as I did three days later when I forgot to reinstall the bag for a training ride having taken it off at Reston the day after Poolesville.
Sign up for Lost River, time is slipping away. The Cat 5 race is going to fill up, the Cat 4 might too. The other fields will have room but you'll save $12 by registering online. Why are "day of" fees so high? Because it's a pain in the ass to register people day of, and I want to encourage people to sign up early so we avoid that. How nice is it for the guy running registration to have his entire registration book and number deal sorted out when he wakes up on the morning of race day? So nice. Making start lists is easier, blah blah blah. I've run registration at our cross race the last two years, and a ton of people decide to go or not go day of for cross races. I understand that some people like to make a game time decision, that's why we offer that option.
The weather looks perfect for Saturday - 80 and sunny.