I'm going hiking for a week in Iceland this spring. What kind of pants would be right for cold and rainy, but good for dry weather, too?
There are two ways to approach this problem, Tom. The first is to find a pair of pants that does everything you need. The second is to take a hybrid approach.
The hybrid looks like this: You get some light rain pants, such as REI's Taku Pants ($159), which are waterproof-breathable and have some stretch. Or, Marmot's Minimalist ($160), which uses Gore-Tex PacLite. Then under that you wear a pair of light tights—Patagonia's Silkweight Capilene bottoms ($45) are ideal in all but very cold conditions. That combination keeps you dry and warm enough. Then, if it's dry, take off the pants and put on a pair of light hiking shorts.
There are some pants that, by themselves, cover most of those bases, offering decent rain protection with a wide temperature range. Probably the best, in my view, is the Arc'Teryx Gamma LT Pant ($169). It's a soft shell made from a stretch fabric that sheds light rain, offers excellent wind protection, and is comfortable from freezing temps up to the low 60s. They're also very tough pants, and can take a lot of groveling around in the mud.
The problem with one pair of pants is that, for an extended trip like a week, you might still want/need some backup. If the weather looks to be cold, then add a base layer. If it looks wet, a rain shell. April-May is the start of the season in Iceland, so apt to be cold AND wet. So maybe pants and light tights and a shell. Shorts probably aren't needed.