The Chao Phraya hotels — Mandarin Oriental, The Peninsula, Shangri-La and Capella — are the choice when you want the river as the view. They pair easily with Grand Palace sightseeing or ICONSIAM shopping, and because several are reached by hotel shuttle boat from Sathorn Pier the journey becomes part of the afternoon. The Authors’ Lounge at Mandarin Oriental is a special case: it occupies the hotel’s original building, so the room itself is the reason to go.
Around Lumphini, Langsuan and Siam the appeal is convenience. Sindhorn Kempinski, Siam Kempinski and Waldorf Astoria sit in the middle of the shopping district, and a chocolate-buffet format such as Chocolab at SO/ Bangkok is a completely different proposition from a classic tea tray. Most venues run fixed sittings between roughly 2pm and 5pm and many apply a smart-casual dress code, so confirm the current set, sitting time and requirements for your date.