there are loads you can try depending on whether you want tall ones like delphiniums, lupins, perennial sunflowers etc. Some good medium ones like maltese cross, rudbekia goldstrum, columbines, geums etc. Good short spready ones are pulmonaria, perennial geraniums are always great and reliable. A lot of these can be grown from seed, if you don't have a greenhouse, try them on the windowsill in a seed tray with a plastic lid on. You can get these cheap propogators from homebase or wilkinsons. You are a bit late for this year, although you could try just a few now and if they grow, plant them out and see if they survive the winter. Best time for seeds generally March/April. You can of course buy plants,( a pricier way of doing it) or scrounge bits from friends and neighbours. Suggest a visit to your local library gardening section and just get a book on perennials where you will find hundreds of ideas. You could try a clematis up the tree as they like their roots in shade but flowers in the sun. I have a jackmanii clematis growing up a tree and it looks good in summer. Good luck.