I've promised my 6 year-old daughter a special vacation week trip to Dunkin' Donuts. And yes, there are probably half a dozen outlets within walking distance of our Coolidge Corner home. But I'm looking to make an expedition of it -- she hasn't had a donut for a couple of years, she tells me -- and would appreciate nominations for Boston's most exciting donut shop. Please keep in mind that exciting, for a kindergartener, might be any of the following: bar stools, fancy lighting, views of donut-making or other activity. I've thought about Union Sq. Donuts, but it looks like there is no seating(?).