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.
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