Hi Patti,
I believe that the last entry into the Grand Palace is 3.30pm but it doesn't actaually close until 4.30-5. It opens at 8.30am.
It is really the "must see" if you are into astonishing achitecture. It is not of the same scale as Beijing' s Forbidden City, but more diverse and interesting IMHO.
A private guide would be a good idea in the limited time you have, though with a little research, Bangkok is reasonably easy to navigate on your own.
I have used Tong for trips out of Bangkok and find her excellent and most of her associates are also highly recommended. If you can get Tong herself, strap yourself in for a "full-on" experience, but make sure you bring your sense of humour and energy to burn.