Hi Podgirl, I am going to suggest a 'virburnam bodnantense' Dawn', which bears beautiful pink/white and highly scented flowers early in the year. Also a bed of lavendar for its delicious scent too. How about Buddleias too, great flowers and they attract butterflies.

I lve the perfume of 'dawn' Gran.....
If it can only be one tree there are some lovely Acers with pretty leaves and autumn colours, also some have unusual bark as well, different textures that are attractive in the winter

I am also gong to suggest the 'prunus serrula 'Tibetica') which is a small tree with outstanding shiny mahogany red bark.The bark gets shinier by being touched.

what about a spring flowering cherry tree like kanzan or accolade?lovely to sit near on a mild spring day.