Two weeks ago an experienced researcher asked me this question. I was speechless. I am not sure the other understood that I was more wondering where to start, with so many things I want to get done, or whether I had literally no idea how to spend that other than propagating my current post-doc position.
Thoughts that blasted through my mind in random order: make the CDK 1000x faster, finish the orbital development kit, make a chemically decent Connectivity Map, run weekly, untargetted metabolomics on my body fluids for one full year (and three months) - that is about where my opponent began wondering if I had any ideas, I think -, do the same for the plants in my garden, do nanoQSAR (and in fact, I am writing this up for 4 million as we speak), develop a linked data, CCZero PubChem/ChemSpider alternative, make a database with the original literature on the first 1M organic compounds ever discovered (starting with ureum)... seriously, I have many more ideas; don't get me started on doing boring things like studying a single disease; we have M.Sc. students for that.
Five million? Bring it on!