depends if you want flowers or just leaves, acer maples are lovely for leaf shape and colour or something like laburnam which has cascades of bright yellow flowers, but be carefull if children as the seeds are poisonous, rown trees are lovely to with red berries which the birds love, so many really, if you put small trees in search you will see lots and tells you sizes and if it flowers or berries, or just leaves

Crikey, where do you start?

As said above, depends on various characters that you may want the tree to display such as flowers or just form and foliage.

I'll stick my neck out and plump for just a couple of literally thousands of potentials....

How about trees with good form and great autumn colour or flowers? Nyssa sylvatica, Quercus coccinea or Liriodendron tulipifera would certainly fulfil the first. Davidia involucrata would the second and it has an almost perfect cone-shaped crown.

I know its not a tree, but it can grow into a great big shrub how about a "witch hazel"??