"There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown". It doesn't say the children were giants; it says they were men of renown. The "giants" reference could mean anything; and when read in context, actually indicates beings or creatures which existed BEFORE the angel/human matings and those offspring.
Now THOSE (the giants) could have done just about anything, I suppose. Maybe even built the structures that originally stood where the Pyramids now stand.