Here's a list of trees that offer seasonal interest. I think all of them will grow in clay soil, though some of them will need improved clay, or clay that is naturally clayey-loamy or sandey-clayey, not 100% clay.

FALL INTEREST:

Chokeberry (Aronia arbutifolia) berries

Japanese Barberry (Berberis thunbergii) berries

Beautyberry (Callicarpa americana) lilac-colored berries!

Cotoneaster (Cotoneaster spp.) berrie

Burford Holly (Ilex cornuta 'Burfordii') berries--there's a standard one and a dwarf one

Oakleaf Hydrangea (Hydrangea quercifolia) foliage

Yaupon Holly (Ilex vomitoria) berries --standard height and dwarf cultivars available

Nandina (Nandina domestica) foliage--many colors avail. from yellow to red to purple

Firethorn (Pyrancantha spp.) berries

Sumacs (Rhus spp.) foliage and berries

WINTER INTEREST:

Flowering Quince (Chaenomeles spp.) flowers in very late winter to early spring

Oakleaf Hydrangea (Hydrangea quercifolia) bark

Holly (Ilex spp.) berries and bark

Winter Honeysuckle (Lonicera frangrantissima) flowers/scent

Oregon Grape Hollly (Mahonia aquifolium) flowers

SPRING INTEREST:

Forsythia (Forsythis intermedia spp.) flowers

Hydrangeas (Hydrangeas macrophylla) flowers

Mock Orange (Philadelphus coronarius) flowers

Bridal-wreath Spiraea (Spiraea prunifoia) flowers

Baby's Breath Spiraea (Spiraea thunbergii) flowers

Vanhoutte Spiraea (Spireae vanhouteii)flowers

Lilac (Syringa vulgaris) flowers

Snowball Viburnum (Viburnum 'roseum') flowers

Viburnum spp. flowers

SUMMER INTEREST:

Glossy Abelia (Abelia grandiflora) flowers

Butterfly Bush (Buddleia davidii) flowers

Bluebeard (Caropteris x clandonensis) flowers

Hydrangea (Hydrangea spp.) flowers

Chaste Tree (Vitex agnus-castus)-- prune for tree form or let remain shrubby flowers

Dwarf and weeping crape myrtle cultivars

More lists to follow.

Dawn