Udon Thani

The lotus bloom runs roughly December through February, peaking in January, and the flowers open in the morning, closing by midday, so go at dawn. Longtail boats leave from Ban Diam pier (about 45 minutes from the city): ฿300 for a small boat, ฿500 for larger, per boat not per person. Weekdays are far quieter than Thai-tourist weekends.

Yes, and it's the best day trip in upper Isan. Trains and minivans cover the 55 km in about an hour. Sala Kaew Ku's giant concrete Buddhist-Hindu sculptures are unlike anything else in Thailand, and the Mekong promenade at sunset, looking across to Laos, is the region's defining view. Many travelers stay the night in a riverside guesthouse before crossing to Vientiane.

Flights from Bangkok run all day (~65 min, from ฿800 on AirAsia, Nok Air, Lion, and Thai Airways). Udon has upper Isan's main airport. The overnight sleeper train from Krung Thep Aphiwat is the classic route (~9 hrs, second-class berth ฿800–1,000), continuing to Nong Khai, and it's the one I'd take. Buses take 7–8 hours.

Yes. The First Thai–Lao Friendship Bridge at Nong Khai is 80 km away, the busiest land crossing between the countries. Direct international buses run from Udon Thani's terminal to Vientiane (~2 hrs), and the through-train continues from Nong Khai across the bridge. Most nationalities can get a Lao visa on arrival at the bridge, but check your embassy's current rules.