grey days are ideal for moving water....it forces you into long exposure times and using a tripod :) grey days are a common thing here in the pacific northwest and i love landscapes myself.... we have many waterfalls here and i find that the light is much more even and easy to handle for long exposure times....so i would suggest using the light (or lack thereof) for exploring the long exposure photos...i.e. moving water, moving grass, people movement etc. etc. good luck :)