I was on a business trip yesterday - had to go to Merced, CA for a short meeting. Wrapped up around 4:00, so having never been to that part of CA before, I headed out to see what I could see of Yosemite before the sun went down. A 6 AM departure this morning and the desire to get a few hours of sleep had me on a pretty tight schedule.
Yosemite was awesome. The small corner I saw was amazing, and I will be back. With my widest lens being a 28mm (wide end of an old AF 28-85) on a D80 (=42mm) I'm really not equipped for landscape work - my results capturing the sights fall more into the snapshot category. Still, I came away with a few shots I'm pretty happy with. A wide lens has also moved up on my shopping list.
2 hours in Yosemite is a heartbreaker. When I go I am always left scheming ways to an early retirement so I can just never leave. I can see myself deciding in that situation to stay up all night and just sleep on the plane the next day.
The photos are nice and I especially like the last one.
Hopefully you get a chance to go back soon. A 12-24 lens would fill in your gap nicely, but barring that, a used 18-55 would be really cheap (<$100) and would open up a lot of possibilities.
If the Sigma is in your price range then that's a better option than the 18-55 IMO. For landscape the 10-20 will open up a whole new world the 18-55 isn't quite capable of.
FYI Tokina makes a well respected 12-24 f/4 that costs about the same as the Sigma. I use the Sigma 10-20 myself and in most cases I recommend it over the Tokina for being able to go 2mm wider, but they are both excellent lenses and in your case the 12-24 does a better job of covering the gap to your normal zoom.