I had one full day in Bangkok and decided to try to find a private guide, so I didn't have to deal with a group tour schedule and could do the things that I wanted to on my own pace. I was paired with Gwen. I appreciated that she came a little early and we chatted before taking off about the things I wanted to prioritize seeing, what kind of commentary I would like for the tour, how long I wanted the tour to last, as well as any medical or dietary concerns. Her English is very easy to understand as well. She was especially helpful when I realized that my debit card went missing (I had accidentally left it in the ATM before her arrival), and when we could not find it or get the bank to retrieve it, she was so helpful in assisting me with finding a Western Union so funds sent to me could be accessed. We went to Wat Sa Ket (it's open earlier than the others) first, Temple of Emerald Buddha, Grand Palace, Temple of Reclining Buddha, had lunch at her recommended place (it was a small cafe but really lovely food), walked through a flower market, and then a walk through the artsy parts of Talat Noi. I thought it was a great mix of cultural, historic, and modern and I really loved the different forms of transportation that we took, ranging from taxi, Tuktuk, subway, to scooter taxis. Even with all of this I didn't use up the entire 8 hours, but I was pretty tired and ready for a long nap after a hot day of touring. I forgot to mention also, Gwen is a FANTASTIC iPhone photographer and I have a ton of really flattering and wonderful pictures throughout the day.
If you're short on time, or you need a good introduction to Bangkok and how to get around as a solo who has never been there before, I'd strongly suggest booking a private guide - and especially look for Gwen!