First off, the Grand Palace has a dress code that prohibits sandals, flip-flops, tank tops, shorts, and short skirts. All of these guideline are strictly enforces except for the sandals... I actually saw many people walking around in sandals and would choose to wear them if I ever make it back again.
My girlfriend and I walked to the Grand Palace from our B&B (The Four Inn) in about 10-15minutes. As we drew near, we were approached by many people trying to sell us long pants, but we were already prepared (see tip#2 below), thanks to the advice of our guide books. We arrived at 9.30AM on a weekday, and there were quite a lot of people there already. However, it was not overly crowded and we were able to make our way through the complex unhampered. There is a lot to see here, so be prepared to spend at least two hours at the Grand Palace. There are so many distinct buildings, temples, and statues, I feel we could have spent the greater part of a day just walking around had it not been for the hot summer sun beating down on us. The architecture is very detailed and unique, with lots of bright colors (especially gold). Make sure your camera has plenty of memory and battery power, as it will undoubtedly get quite a workout here.
Tips for the Grand Palace:
1) Beware of "tour guides" telling you that the palace is closed. They're only trying to get you to jump on their Tuk Tuk so that they can take you somewhere else for a commission.
2) You must wear long pants or dresses when visiting the Grand Palace. I brought a lightweight pair of pants that allow me to zip off the legs and convert into shorts. These were perfect, as I walked to/from the palace in shorts, then explored inside with the pants.
3) I wore tennis shoes because of Lonely Planet's advice to "dress conservatively", although I saw LOTS of people wearing sandals. I would advise all visiotrs to also wear sandals, as the tennis shoes were extremely hot...
4) Lonely Planet also advises visitors to wear long sleaves, which I did. THIS IS NOT NECESSARY. Short sleaves, and even t-shirts, are fine. Tank tops, however, are a no-no.
5) Go early, as the area fills up rather quickly. Also, the heat is a big concern as you will be outside in the sun for the majority of the time.
6) Bring LOTS of water... although you can buy bottled water at the palace if you're in a pinch.