We are making our first trip to CR at the end of January, doing the self driving trip to include with a guided Arenal hike, other hikes, going to the central Pacific coast and back to MV. In Colorado we hike many of the 14ers in just tennis shoes. Are hiking boots necessary or a better idea than good tennis shoes for the volcano? Other apparel besides the obvious (rainwear, cool weather, hats, shorts) which can enlighten us is appreciated.

I just wear good running shoes, and that's always worked for me. I also bring Keen's for areas that are wet. If your shoes get wet, it can be hard to dry them. In addition to everything you mentioned, we always pack a pair of binoculars as well.

I haven't been to CR but for general travel to CA I prefer wicking clothes - cooler when it's hot, layer well, rinse out and dry in a flash. Some clothes that are called wicking are cheap knock-offs. The good ones are spendy but last for years. I figure any products REI sells are the real deal but I wait till they're on sale to buy; I've listed my favorite online resources below.
www.rei.com
www.rei-outlet.com
www.campmor.com
www.sierratradingpost.com

Unless I'm planning heavy trekking I wear Keen H2Os and pack along flipflops, that's it for footwear. My packing list is #14 in the FAQ thread here:
http://www.lonelyplanet.com/thorntre...hreadID=828622

Happy trails!

Tennis shoes are fine, but better if you have your hiking shoes!

I think hiking boots are better for around the volcano but I'm sure you would be fine in tennis shoes.