Like a lot of people I know, my brain works on coming up with solutions to 'problems' 24/7, whether I want it to or not (much to my spouses dismay sometimes) And while effective getting those solutions out of the queue require either isolation (camping is great, the shower is ok if a bit short) or to have my conscious brain not focused on trying to pull the answer out.
Juggling (and as it turns out for me piano playing) has the property of applying a temporal focus to my brain which prevents it from interfering with my subconscious. Weird I know, but folks I know who study such things consider it fairly normal. I know a guy who plays FPS video games when he has a really hard problem he's working on, same sort of thing.
This has been a real issue for me and the fact that my off-focus activity of choice is arsing about on the internets and pretty much everyone I ever work with thinks all I ever do is slack off.
I've replaced the driving with biking (I live a lot closer to my job now), but it honestly isn't quite as good. Some of the stuff I came up with then (this was about a year and a half ago), I'm still fleshing out now.
Another thing that's close to driving, but not quite as good is yard work. Something about the sound of lawnmowers and weed whackers is really conducive to thinking.
No distractions, and if I needed a break to let something turn over in my mind, I could simply step out of the car watch the ocean, or go up to the passenger decks and have a seat or get a snack from the cafeteria. If it was the morning, they had nice breakfasts available.
I'd be curious to see a bit more detail on the physiological basis of this. I do plenty of things where you're able to zone out a bit, but none of them have remotely the same idea-generation rate as being in the shower. Can't help but wonder whether a big pile of warm water all over the body does something to blood flow in some useful way. If I could remember any physiology this would be the point to use an impressive word like vasoconstriction or vasodilation or some such. :-)
My reply to your remark was placed under the wrong comment (that's what I get for posting at some ungodly hour while suffering insomnia):
I realized recently that what one thinks about in the
shower in the morning is more important than I'd thought.
I knew it was a good time to have ideas. Now I'd go
further: now I'd say it's hard to do a really good job on
anything you don't think about in the shower.
That's so true. My morning shower is where I think about what my goals are for the day- it's also usually where and when I come up with the seeds of the best strategies for achieving said goals. If I haven't found a executable solution by the end of my shower, it usually won't come to me until 3am later that night when all other distractions are gone.
(Also, I seem to be really sparse in the idea department. Or rather, in the interesting idea department. I've spent most of the last decade stumbling onto ideas that I could immediately dismiss as being too obvious to be interesting, whether or not they could be financially rewarding, or already done by someone else. There needs to be some sort of affordable R&D/think-tank outsourcing you can do if you're a one or two man job looking for ideas - heh.)
It's not about finding an interesting idea fully formed. It's about constantly generating new things to try, new trails to blaze.
"As if I don't have enough things on my to-do list."
If it's closing you to new ideas your todo list is too much baggage. I've grown super sensitive to this; I prune my todo list all the time.
It's hard to be open to new ideas all the time, of course. Being too open can interfere with actually doing anything. The author of "Flow" says successful people tend to alternate between being open and exploratory, and being closed/focussed on getting something out the door. I imagine it as weaving alternately on either side of the ideal line. It's impossible to perfectly balance both sides, but you don't have to be perfect.
 My todo list is in version control for many years now, and every now and then I check out what it looked like a month or a year ago. It's amazing to me how rapidly items turn over. Most ideas form and die within 2 weeks.
Why should it not be interesting just because it seems to be obvious?
In college there were multiple times that I came up with solutions to programming problems while I was in the shower because my mind actually had time to take a break.
I don't know if that's completely random, whether it's something specific about me, whether it's because I will spend longer in a bath than a shower, or because I enjoy having a bath more, and tend to relax more. Or some other reason.
The key is that the activity has to be both relaxing and routine (double "r"...). For you, it's bathing, for others it's showering, yet for others it's walking in the woods. If the activity is not relaxing, you will be preoccupied with how to control it and with resulting stress. If the activity is not routine, you will be focused on its novelty rather than letting your mind wander on its own.
It's just that in a shower I never decide to relax and enjoy it, it's always a method of cleaning, or of waking up, or of heating up, not an activity for the sake of enjoyment.
My best idea generator is the walk away from my desk & to my car at the end of the day. I have solved so many bugs immediately after exiting the office building...
Get one of these, and jot your ideas down in the shower. (Don't forget to buy cleaning fluid: http://www.amazon.com/Biodegradable-Dive-Slate-Cleaner-Refre...)
I might be able to take an hour for a walk around the neighborhood to clear my head to get ideas, but generally even walking around trying to not get hit by vehicles or other pedestrians still distracts me.
When you're taking a shower, this is often the only time during the day when you're not actively engaging your mind. Just taking a walk (no iPod allowed) or just lying down on the couch staring at the ceiling, or sitting in your car after you've parked will give the same effect. Guaranteed. You might experience a few minutes of discomfort as your consciousness grasps for something to hold on to.
Taking showers to mull over problems is one more advantage to working from home :)
Also, get Pragmatic Thinking and Learning by Andy Hunt. Also covers these topics and helps people understand learning, creativity and how to exploit the way you work and think to, well, learn better.
http://www.simplynoise.com and also their downloadable thunderstorm soundscape recording: http://simplynoise.com/download.php?key=18336dd65313 (Google result, not sure why it thinks a donation has happened).
[also, maybe i'm just jaded, but the way i read your reply sounds as though you're indignant - as if i've committed some terrible (almost racist) offense by making assumptions about other people. if you are offended, i apologise, but i mention it more because i wonder if it's just a curious consequence of the kind of society we now live in. i have no idea how you would phrase what you said to avoid giving that idea, for example...]
There isn't too much going on to distract me.