This is my suggested itinerary, if you have at least 10 days to spare for Thailand. It can be arranged and rearranged according to your wishes and flight plans:
Day 1 – Bangkok – Start your day with cultural sightseeing. Head to the MRT station Sanam Chai, walk to the Tha Thien pier and take a ferry across the river to visit Wat Arun (this wat gets crowded later in the day, in the morning it should be better), then take a ferry back and visit Wat Pho (I believe it’s crowded any time of the day unfortunately, but after people leave Royal Palace in the afternoon – even more). After both Wats, head to the Royal Palace (keep in mind, it is open until 3.30 pm). The palace is always extremely crowded, it was the most crowded sightseeing spot I have visited, I think. See the detailed tips for visiting the Palace here.
After all the sightseeing, hop on a public ferry at the Tha Chang pier. There are several ferries in different colors, only one, so called “tourist ferry” is more expensive, others cost 15-20 baht. Finally, after going up and down the river, get to Sathorn ferry stop, which is near the BTS station Saphan Taksin. In the evening, you can visit one of the numerous bars with views, such as Sky bar, Vertigo, the Roof, etc.
Day 2 – Bangkok – get to the Sam Yot MRT station and walk to Wat Suthat Thepwararam. It is an underrated place, but very beautiful, there is also a so called swing in front, which no longer has a seat, but still a popular place 😊 Then walk to Loha Prasat, it’s a very interesting Wat, with great views. After this, cross the canal to get to the Golden Mountain. You will need to walk up the steps and it has beautiful views over the city.
After this, get back to the MRT station and head to the Lumphini park. If you are interested in Erawan shrine, it is about 20 mins walk from here. From Erawan shrine, you can hop on BTS and, if it’s a weekend, head to the very popular Chatuchak weekend market (Mo Chit BTS station). You can buy basically anything you need here and it works until 6pm. After the market, or if it’s not a weekend, directly from the Erawan shrine, head either to Siam for visiting the Siam Paragon or Central world mall (next to Erawan shrine) or, if you are interested head to Sukhumvit and visit the red light districts – either Nana Plaza or Soi Cowboy. You can also find numerous malls in Sukhumvit area, such as the newly build Terminal 21, with different city concepts.
Day 3 – Krabi Province – fly to Krabi or Phuket, depending on your preference. I will give suggestions for Krabi, because we chose that one 😊 You can find out about, why we chose Krabi over Phuket and our observations here. We stayed near Ao Nang beach in Krabi province, which was convenient for visiting other places as well. On our first day, we just explored Ao Nang beach.
Btw, you can go with a wooden path across to Pia Plong beach, which was very beautiful and far cleaner than Ao Nang itself.
Day 4 – Krabi province – head to Railey beach by a longtail boat. It costs 200 baht for return tickets. You will get to West Railey beach and can explore this part and head to the Eastern. Eastern side is not great for swimming, but it has some extremely beautiful spots. From here, you can walk to the Phra Nang cave beach. This one is a must, it was my favourate place. See the details of exploring these two beautiful beaches here.
From Phra Nang cave beach, you can catch a longtail boat back to Ao Nang with the same return ticket you bought already. You may have to wait for a short time, so that 8 people are gathered, which is the minimum they take.
Day 5 – Krabi province – you can either book one of the many day tours going to Phi Phi islands (they are rather expensive at 1800-2200 per person) or get a ferry ticket to go to Phi Phi Don (this costs 700 per person for the return trip). We didn’t wanna run around different spots with the tour and it was also extremely expensive, so we opted for going to Phi Phi Don (from where you can hire a longtail boat to take you to Phi Phi Leh to see Maya bay and Pileh Lagoon). On Phi Phi Don the main interesting spot is the Viewpoint. Viewpoint 2 has the best views.
Day 6 – Krabi province – you can relax at the beach in the morning and then head to the Tiger cave. The tourist booths have a number of options, for joint tours, they ask for 350 baht per person, however, we managed to hire a car for the two of us for 900 baht (just ask around). If you are 3 people, it would end up cheaper, than the tour with other people. Allocate at least 1.5 hours to spend at the tiger cave, as walking up 1260 steps (some of them very steep) took us about 40 mins and we are not too out of shape 😊 You will find the detailed info about visiting the Tiger Cave here.
Day 7 – Chaing Mai – fly to Chiang Mai. Visit Doi Suthep national park, including Wat Phrathat Doi Suthep and Bhubing Palace.
Day 8 – Chaing Mai – explore the old city and visit the many wats in town – Wat Sri Suphan, Wat Phra Singh (Gold Temple), Wat Chedi Luang, Wat Phantao, Wat Chiang Man. Head to Chiang Rai.
You also have an option to visit Elephant ‘sancturies” around, but they don’t treat elephants nicely, they let people ride them, etc.
From Chiang Mai you can visit Pai as well, many people love it.
Day 9 – Chiang Rai – Visit the White Temple, Blue Temple and the Black House. Fly to Bangkok
Day 10 – Ayutthaya – take a train or a mini-bus from Bangkok to Ayutthaya. Visit the numerous temples there. Don’t miss the Wat Ratchaburana, Wat Mahathat, Wat Phra Si Sanphet, Wat Yai Chaimongkhon. Other interesting temples include: Wat Phanan Choeng Worawihan, Wat Phutthai Sawan, Wat Chai Watthanaram, Wat Phra Ram, Wat Thammikarat, Wat Phu Khao Thong. Some of these temples are central and you can walk among them, some are farther away and you will need a bike or a tuk-tuk for them.
There are other interesting places in Thailand to go to, if you have more time and interest in the country. Pattaya is a very popular option. I personally didn’t enjoy it, so I didn’t include it in this itinerary, but you can check it out, if you are interested. Other popular destinations include Kanchanaburi and the Death railway, Erawan waterfall, the numerous islands in south of Thailand, I have been suggested Koh Phangan, Koh Lipe, Koh Samui, etc.