When I'm mountain biking I just use a kit lens: a Nikkor 18-55mm. Plastic and feels like a toy but does a great job - unbelievably sharp for such a cheap lens.
Since I'm always shooting landscapes, these are always in my bag. I use a ultra thin Marumi DHG CPL which is always attached to my Sigma 10-20mm. It serves as a lens protector also as I don't use a UV filter. Gotta avoid vignettes when shooting wide, especially when I'm stacking filters.
Aside from the CPL I bring along a Hoya ND8X, an ultra thin Haida Infrared 720 filter (equivalent of Hoya R72) and several Cokin P-series GND filters. I tend to bring soft and hard edge GND8 filters and a GND4 most of the time. I stopped using colored graduated filters like the blue or sunset filters. I'd like to get my hands on 'em Singh-Ray filters one of these days. A single Singh-Ray filter costs an arm and a leg and a good portion of your torso.
AccessoriesLittle stuff I can't do without. I bring several SD cards of different brands. On a typical 3-day weekend trip 4 extra cards usually suffice for my needs. I cycle through 3 extra batteries (the most I can bring is 5). If I bring along my flash gun then I also bring 8 AA rechargeable batteries.
Other stuff includes: commercial grade lens tissue (that can be used for cleaning sensors as well), several lens pens, a blower, camera rain covers, a small but ultra bright torch, a small compass, chargers for camera and AA batteries, a set of RF receiver and transmitter for remote flash, and IR remote trigger.
LightingCurrently I have only 1 flash gun: a Sigma EF-500 DG SUPER (Sigma's answer to SB-800). I use small Lastolite light modifier which is great for outdoor portraits.
If there is a significant source of natural light I also use a disk reflector. There are 5-in-1 versions you can find in most camera shops. It has gold, silver, white, black, and translucent parts.
SupportA tripod is a landscape photographer's best friend. I don't leave home without it. Nobody has rock-steady hands.
My Benro A-157 with a BH-1 ball head has seen its share of rivers, lakes, waterfalls, beaches, mountains and rocks, and has the scars to show for.
Dry bags or dry sacks are essential especially if you are trekking in the storm and crossing rivers and lakes. I use a 4-L bag and a 25-L sack. I use the bigger sack for my camera and lenses; the smaller one I use for the accessories.
I also bring along several of these small dehumidifier kits that can be bought from hardware shops. I put one inside my camera backpack and another one inside the bigger dry sack. I try to keep my gears as dry as possible whenever I go out and shoot waterfalls.