I guess be prepared for weather extremes. The day before I hiked Volcan San Salvador, a German traveler did it. He told me it was warm and sunny. The next day I nearly froze up there with gale force winds and clouds whipping over the rim.
So, dress for cold and hot, I'd say. Sturdy boots, long pants for the rockier or rough parts, hat for the sun...you know the normal drill. And a good guidebook could probably tell you if those volcanoes are active. Ones that are fairly active tend to be big tourist attractions (like Arenal in Costa Rica) and would be mentioned...
worldwidemike