You can do some very pleasant walks around the Royal Docks (there's a very odd bridge in the middle of it) and down to the Thames Barrier Park for view of the, well, you can guess. A trip on the pointless-but-impressive cablecar over to the O2 is a possibility too, depending on the amount of time you have.

So would I. Particularly given how many times I stood waiting (for an annoyingly infrequent train) on that forlorn platform at the old Silvertown station!