Green and indoors
A half-day with children has fewer queues and fewer steps. Lumphini is flat, or Benjakitti's lake; at noon, Jim Thompson (the house has stairs and no lift — decide first) or Museum Siam. The snake farm's talk follows the institute that day. With a stroller, confirm the station has a lift. Boat and canal steps usually do not.
What to skip
What to skip: the long Jay Fai queue, Chatuchak at Saturday noon, Khaosan and Soi Cowboy, and Wat Arun's steep stairs as the only activity. If you do the market, move to earlier on Sunday and agree one direction before you enter.