One thing you can do is raise the crown of your trees to allow in extra light and water. This simply means removing the lowest branches to expose mor etrunk and then thinning out crossing branches inside the crown to show of fthe structure of branches and let more air flow.
After that, the choice of plants will depend on how exposed your garden is to wind and frosts and the type of soil you have. If possible, improve teh soil by forking in a good amount of well rotted garden compost and/or manure and then add a thick mulch of teh same stuff for the worms to work in over winter once you've planted and watered.
Plants to consider include hardy geraniums macrorrhizum and phaeum which do well in shade and dry soils, euphorbia amygloides "Purpurea", iris foetidissima, forms of lamium maculatum and vinca minor.
All of these have flowers and some have variegated foliage. You can look them all up on the RHS Plant Selector - http://apps.rhs.org.uk/plantselector/