Agree with all the 'less is more' comments but would emphasize that extra batteries are a good idea...Almost a must if you snap a bunch and/or use fill-flash frequently. My 'essential' lightweight setup includes the following:
(1) GGS 3" screen protector (minimal degradation and preserves the screen)
(1) SanDisk Extreme 16gb SD card (the 30mb/sec version but would recommend the newer, faster ones)
(2) extra Maximal Power NP-50 batteries
(1) 40-46mm step-up ring with B+W MCUV 46mm filter (always on the camera)
(1) each B+W ND8 filter, IR 690 filter stacked in a metal filter cap
(1) Lenspen and cleaning cloth (stored in a small ziplock bag)
Everything above fits comfortably in a pair of cargo pants/shorts. When wearing jeans or dress pants, those items will fit but I often find them contending for room with my wallet, keys, phone, etc.
The camera itself is usually wrapped in a Gariz half case (adds height to the camera which I like for my medium sized hands, didn't get it for protection...I mean it only covers half the camera!), Gariz thumb strap (don't like how the thumbs up solutions cover the hot shoe) and occasionally the Gariz Gun Shot strap (when not housing the camera in my Crumpler 2 million case or messenger bag).
When I am toting a larger bag like my messenger, I'll bring my EF-X20 flash (get the EF-20 if you don't value/need the size, build or wireless capabilities of the X version), Canon knock-off TTL flash cable (for off-shoe use), WC-E68 wide-angle adaptor (makes the X10/20 a 19mm lens, also a 46mm thread), my old Biloret super compact tripod (for the camera), small gorilla pod (for the flash) and PowerGen Rapid Smart Charger (charges batteries for all my X-Cams, iPad and iPhone via AC or DC). And of course a variety of cables like USB, HDMI, etc.