I would suggest Laburnum. The foliage is pretty, it has good flowers and best of all it does not shade out what is underneath. Another suggestion would be Hawthorn for it's flowers and later berries.

For something a bit more unusual, why not try a ginkgo biloba ? These are really lovely small trees and very, very slow growing, with unusual, waxy feeling leaves that turn bright yellow in the autumn.

Well Lollipop, it really depends upon the type of look that you want to achieve in your garden and if you want an evergreen or not.
I would personally suggest that in full sun, a fatsia japonica that will have nice shaped evergreen leaves, is frost hardy and will give a tropical touch would be ideal. Although usually just kept as a shrub, it can be very easily shaped into a tree by removing the lower branches and growth. I have some beautiful specimens in my garden.
If you don't mind them losing their leaves, most of the Cotinus will give you some of the most beautiful small rounded leaves, that will go from yellow/lime green to orange or deep red. They will need to be in full sun for the best colour of the leaves. They are small trees which will grow to about 5 metres and can be easily shaped.

maybe a twisted willow, mine is about 20 years old and at most 15 feet tall, has unusual form,offers dappled shade. Unfortunately it is deciduous

By some definitions, a small tree is any one that matures at less than 6 meters tall and wide--probably still too big for a really small garden. It sounds like you are looking for a large shrub, maybe trained as a standard (aka "patio tree").

Depends what you mean by 'small garden' Lollipop. Two of the trees mentioned here are pretty large in our garden which is 40feet by 50feet. We have pruned bits off them over the years but they still manage to assert themselves. The most controllable, decorative 'tree' for me is a nice Camellia, if soil and aspect suitable. Other alternative is let it grow till it starts getting bossy then remove and start again.