I like the lacecap hydrangeas, myself (link below).

I always have to smile when I hear someone say that more trees need to flower in late summer/fall.

The purpose of flowering is to produce seed. They usually flower in spring so that seed has the time to grow to maturity. There may be some trees that flower in late summer, but their seed probably doesn't ripen until the next year.

Trees do what they do for themselves, they don't care what we think.

Sue

Here is a link that might be useful: Lacecap hydrangeas

What about the Sourwood Tree? Oxydendrum arboreum.
I have two. One is doing just fine. The other has been growing rather slow for years. I may move it after the leaves drop this Fall.

As far as Hydrangeas go, 'Weiderspink' is my favorite.
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Followup is on same site.

Here is a link that might be useful: Article - Trees that merit planting in Seattle by Arthur Lee Jacobson

There are a few.......:-) And if you add in those shrubs that have tree-like proportions or habits, that number increases.

Sourwood, Oxydendron arboreum, is an excellent example. I'd add to that Stewartia, Albizia, Magnolia grandiflora, Chionanthus, Koelreuteria paniculata, Tilia tomentosa. More shrubby examples are elderberries, Heptacodium, Buddleja, tree-form hydranges or Rose of Sharon, Eucryphia, Aesculus parviflora, smokebush and chaste tree, Vitex agnus-castus.

Thanks for mentioning Eucryphia. My one threadbare-in-the-upper-regions because of two consecutive tough winters has leafed out acceptably and all the others are in fine form.

Oxydendron and Stewartia are both great but they want lots of moisture and the former is intolerant of heavy soils. Another idea is Hoheria, either Hoheria glabrata or H. populnea, both blooming in summer and nice all around trees.

As for Hydrangeas, there are a lot to choose from but something important to consider is size. A plant like 'Beaute Vendomoise' can top out at over 6 feet while 'Lanarth White' might be half that. Other than that it's just a matter of looking at the pictures and picking what you like best.