Hi, the arrival into Bangkok at 6am is pretty painless, the hardest part is the long walk from the arrival pier to Immigration. At that time you will find a line of 3/4/5 people waiting to get through and your turn will come up in minutes. Anyway at this time of the morning, nothing is open so there is no hurry.

Contact ratservice, there would be hundreds like her, just search, but she or someone else will pick you up at the airport and drop you back later that day. Early morning it is lovely to see the flower market and to get into the middle of Chinatown, to sit in the grand Shangri La hotel foyer lounge, be served tea and watch the river traffic go to and fro. The Grand Palace opens around now so an early start there. After the palace it is time for lunch, if your wife would like a spa/massage there are thousands to choose, I notice you are a Marriott, and that hotel has superb facilities. The car and driver will stay with her all day, they find a shady tree and have a snooze. Central Bangkok is full of up market malls, one after another, so if she likes shopping a couple of hours in Centralworld, Paragon, Platinum will delight her, or the girls love a new one call Terminal21

To answer some of your questions, whilst it is easy to walk around the palace, I would recommend you have a guide. The train into the city is simple, but does she have any carry on?, taxis are scarce around the rail, but the street is full of them. A car and driver guide, plus all the expenses of a day in Bangkok should set you back $200, a massage will cost $80 in a hotel, only $20 in a mall, so exchange enough into Baht