The only Rhododendron, I would ever consider planting is the deciduous R.luteum. It has yellow flowers and the most delicious scent in spring, it may also (depending on your soil and the weather) give autumn colour as the leaves fade. Another suggestion is spindle berry, it is an understory plant of british woodlands and has the most incredible pink seedcases with orange fruit inside, which hang like miniature lanterns from the thin branches during the autumn and winter. For deep shade go for something like Euphobia 'Robbiae', it can be invasive, so be sure to only plant it where it's growth will be restricted by poor light levels - i.e. in deep shade, another plant which would work in similar conditions is variagated ground elder.
Very nice to have snowdrops already established, you might also plant some winter aconite, Eranthis hymaelis, to compliment them. This has a yellow flower with a ruff of green bracts surrounding it. There are various grasses/sedges that will thrive in shade, Luzula syvatica, or Milium effusum 'Aureum'. As for shade loving shrubs I would recommend Fatsia japonica for a tropical look, or for a more natural style try Fothergilla major.