25'-30' = small tree, unless you want to plant something that will grow larger than this space. (Big tree = 60'+). As with shrubs and hedging, the small and in-scale ones tend to be not so fast growing, the ones that shoot right up liable to get huge and be coarse or brittle. But even a fast tree will take years to develop. For immediate relief you really need a structure, such as an arbor or lath.

Some possibilities for trees are

Trident maple
Vine maple
David's maple
Japanese maple
Amur maple
Juneberry
Redbud
Fringe tree
Yellow wood
Flowering dogwood
Kousa dogwood
Filbert
Smoke tree
Hawthorn
Dove tree
Snowdrop tree
Crape myrtle
Flowering crabapple
Sourwood
Persian parrotia
Cherry plum
Japanese flowering apricot

You won't find all of these perfect in every way - some are rather slow and others produce large fruits that will drop into plantings and onto pavement in season - and a few might have to be pruned and trained a particular way to fit in the space. And others will not remain small if happy there, eventually growing taller (or wider) than 25'-30'. But all should have potential and are certainly worth investigating.

Fastest growing I've ever heard of is Pawlonia tomentosa, the Empress tree, with fragrant purple flowers. Is said to grow 12 feet per year, the wood is valued.

Here's a site with pix of growth over 3 yrs:

http://www.paulowniasupply.com/ppcPsup/ppcPsup_index.htm

Less spectacular but very pleasing is the Katsura tree - it's grown to 30 feet in ten years in my yard, slender multi-stemmed tree with light green heart-shaped leaves on pink stems, and the whole yard smells of butterscotch in the fall when the leaves turn pink and gold and fall. I'm in love with this tree, but its shadow is slender rather than wide-spreading.

Here's wikipedia on it;
Katsura is the Japanese name for the tree. The scientific name Cercidiphyllum refers to the close resemblance of the leaves to those of Cercis (redbuds); these two unrelated genera can however be distinguished easily as redbud leaves are alternate, not opposite.

The two species are:

Cercidiphyllum japonicum. Throughout the range of the genus. A tree to 40-45 m tall in the wild, though typically smaller in cultivation. Bark rough, furrowed. Leaves smaller, not over 4.5 cm long and 3.2 cm broad; stipules falling soon after leafing out in spring. Seed winged only at lower end. Plants from China were at one time separated as C. japonicum var. sinense, but no consistent difference between Japanese and Chinese origins has been found.
Cercidiphyllum magnificum. Endemic to central Honshû, where it grows at higher altitudes than C. japonicum. A small tree, not more than 10 m tall. Bark smooth. Leaves larger, up to 8 cm long and 5.5 cm broad; stipules persistent. Seed winged at both ends.

bboy's list seems to fit your space very well.

Parrotia, Japanese maple and dogwood would be my favorites on that list.

I almost never plant redbud anymore, because most from local nurseries either have weak unions and split, or develop weak branch unions from homowner neglect.

Filbert is one I've always enjoyed.

River birch might be worth looking into as well.

M. D. Vaden of Oregon