A fellow blogger and writer, Roxanne Sherwood, posted a fantastic entry about writing when you're away from the computer.
I "write" in the shower, while cleaning the kitchen, and while doing laundry. My favorite place to mentally write is in the grocery store. Be forewarned, this can sometimes trigger strange looks from fellow shoppers. When that happens, I realize (too late, of course) that I'm muttering a dialogue sequence under my breath. But you absolutely must read dialogue aloud to make sure it sounds natural and has the correct rhythm. So looking like a crazy lady is the price you pay for writing while shopping, I suppose.
The point is that you should write, even when you're not at your desk, fingers on the keyboard.
If you'd like a great list of ideas for when you can ponder your current work in progress, Roxanne provides it in her blog post, A Time to Ponder. Check it out, and you might realize you have more writing time available than you first realized.