Is it taboo to wear open toed shoes in temples and sandels in the finer hotels? How do you handle this if you are touring in your comfy Ecco sandels?

Open toed shoes are fine. At the Grand Palace?Temple of the Emerald Buddha, you sandals must have a heel strap.

Nice sandals are fine at hotels.

The Temple of the Emerald Buddha and some other royal temples require that people be conservatively dressed. For women, this generally means no bare shoulders, skirts or pants that extend below the knee and as I recall, if you're wearing open-toed sandals you must wear socks (but I think that might be men only). Note that you must remove your shoes entirely to enter any building within a temple, everywhere.

Hotels are usually less picky. Footwear is almost never an issue, but there might be a problem if you try to get into their fine dining restaurants in your bathing suit. ;-)

I always have a sarong with me along with a light weight cotton shirt, since you never know when you may want to enter a temple. Also flip flops or my pumas have always worked for me for casual touring. I always travel with a couple pair of dressy flip flops or sandals for nicer attire, since they are so easy to slip in your suit case.

The signs at the Temple of the Emerald Buddha - Grand Palace Entrance show that sandals must have a heel strap (so no flip-flops). It also shows no cropped pants (capris, etc) though I have seen people in capris get in. No bare shoulders allowed. Note that this venue has the most restrictive dress code of any temple in Thailand.