I am thinking of spending the day walking to Space needle area (don't think we'll go to top) visiting the Chihuly garden area(is it pretty outside there too?) possibly visiting the museum of POP that looks like fun, and walking to pikes place market (or taking monorail) just to wander around.... we are likely to be fairly lazy ..... The underground tour sounds intriguing, seems like a lot of tours available, is it really interesting is my big question.
visiting the Chihuly garden area(is it pretty outside there too?)
About the wheel chair, you should be fine in the market. I suggest you split into smaller groups to explore and set up a meeting place. A train of 10 people isn't going to work well. There are all sorts of people zipping around Seattle in motorized wheel chairs. It may mean a more indirect route. All transit is wheelchair assessable.
An earlier poster is right, you can't really see anything @ Chihuly without going in. The international fountain at the Seattle Center is fun to watch.
is the light rail easy to access from the airport with no car?