Thailand is well connected with modes of transportation like flight, train, bus, ferry and taxi. You can travel from one city to another easily. You will get direct flights between cities like bangkok, krabi, phuket, surat thani, chiang mai, etc. Flights are the fastest way to travel within thailand and can be cheaper then any other mode of transportation. I got my tickets from
Bangkok to krabi Rs. 1326 per person (air asia)
Phuket to Bangkok Rs. 1469 per person (nok air)
Above mentioned prices were promotional. Better you check at the time of booking.
Bangkok is well connected by metro, bus, taxi and tuk tuk (auto rickshaw). Metro is the fastest and cheapest way to travel within the city. If you are travelling solo or couple. Taxi is the good option for families and groups. Buses are also a cheap option but My recommentation is to not travelling by bus. Because bangkok has huge traffic problems. I literally completed a journey of 4.5 kms in more then 1 hour. So avoid bus in peak hours.
If you are a solo traveller or a couple you can rent a scooter on per day basis which varies between Rs. 500 - 600 per day. It is the most preferable mode of transportation by many budget travellers. At the same time it is money and time saving option.
Ferries are also available from certain places to certain destinations like i took
Krabi to Phi phi don 1100 per person
Phi phi don to Phuket 900 per person
I booked ferries online. There are various websites available on the internet.