Quick answer: The best time to visit Bangkok is November–February (cool, dry season). Expect a mid-range budget of about €55 per day. Visa: Visa-free/visa-on-arrival for many passports (30–60 days) — check your nationality.
Why this window
Bangkok is best planned around the river and the heat. This 4-day plan front-loads temples in the cool morning hours and clusters nightlife so you never cross the city twice.