If it is a project I strongly believe in and mostly said we should do, and it is more fun than frustrating, I will get into big focus and work or think about it continually until it is finished, then I slump and relax for a while, going around and bragging to my friends and writing docs and maybe tech talks.

But if it a project without intrinsic motivation or large and clearly present business value (like fixing an incident or saving a $10M client contract but not some hopeful!thing that might pay off in two years), then I keep my weekends to maintain myself as a human being (which ironically also happens to benefit the company on that two year time scale).