Been to Bangkok and Thailand as a whole many many times. First visit over 20 years ago, we worked there as well.
If its your first visit I would say 3 nights in the city, there is plenty to see and get over the flight. But prices are creeping up compared to what you would of go even a couple of years ago for a good quality hotel. Bangkok is not the place it once was for excellent quality hotels at a good price.
In April this year we stayed at the Okura as we like the Japanese brand, not worth the cost, but there are many to choose from, just set yourself a budget. Stayed everywhere from the Pen, Mandarin, Sheraton, Intercontinental....on and on
As far as a beach resort goes depends what your looking for budget wise and your style of holiday, you can either fly or get private transport. Been to many Islands as well as other areas all over the country.
Really think you need to look at guidebooks/Trip advisor/Travel Agent to get a better idea of whats for you and of course your budget and what your wanting out of your holiday, dont know your age but some Islands or geared to different groups..
Thailand has changed a lot over the years, and IMO not for the better and yes it can be like marmite some love it and some really dislike it, but hey ho thats like most holiday places!