Surprisingly, I was able to wash off all of the red dirt of my boots (just used water and a good nylon brush). Socks were ruined for ever. One thing that helps is spraying your boots with waterproofing spray (you can get it in any shoe store), the dirt and water do not penetrate then and can be cleaned off easily. I try to spray mine on regular basis.

As to cooking vs. going out - it is definitely less expensive to cook in as opposed to going out (except local places which are really inexpensive). I ended up cooking quite a lot (especially breakfast, almost daily), we packed lunches most of the time too (saves a lot of time and hassle if you don't have to find a restaurant then wait to be seated and served), dinners sometimes (then you can splurge on a nice bottle of wine to enjoy with your dinner). Kauai has a good number of grocery stores/supermarkets - some better than others, generally better on the north shore or in Kapaa or Lihue than on the south or west shores. Roads are well marked and we never had a problem finding places.
To Marie: the Kalalau trail can get very tricky if it is muddy since it is also steep and narrow in places. As Sarah said, wait for a couple of dry days in a row and then hike it - you should have no problem this way. Sneakers generally do fine for hiking as long as you have ones with good traction.

rph, hopefully someone can chime in re boat tours. I have heard a lot of reports of people getting sea sick but I am sure it also depends a lot on the weather conditions, not only the type of boat you take.

Diana: reserving a helicopter: we found that you really should call a couple of days in advance, otherwise it is booked.