You can choose either basis:
Bhutan - flights are pretty expensive so check in advance, might exhaust your budget.
Nepal - for adrenaline rush and under 30k it would be an amazing trip. Thailand - If you get lucky & book via airasia - you can book flights in under 18k & spend 12 -15k on stay/travel/food. But could be done in a bagpacker way.
See , if you are planning to go to Bhutan by bus from siliguri, kolkata or coochbehar. then 30k is a very comfortable budget. Nepal, again if you are planning to go by road, 30k should be sufficient. But Thailand (as i have been there twice), contrary to rest of the suggestions here , doing it in 30 k is possible with airfare.
Day 1: bangkok: Day 2 : Pattaya
Day 3 : Pattaya
Day 4: Bangkok
Day 5: Return
Cost : return airfare: Rs.9k ( if planned in advance ) , accomodation : 6k ( 4 nights, one gets a decent hostel/hotel for solo travellers for 1.5k per night), food and drinks : 5k ( 1k per day )., transfers and sightseeing: 6k ( 1500rs roundfare bangkok to pattaya, another 4.5 k for roaming around in pattaya and bangkok,entry fees to palace, boat cruise in bangkok etc, avail public transport), visa :4.5k ( 2000thb ).
Keep another 5k extra in hand just in case of shopping or any other emergency.
Find good online deals in klook and viator for exploring around bangkok and pattaya. Book your bus transport from bangkok to pattaya from 12goasia.com. They are a trusted and a good travel agency for south east asia. Find excellent airfares ex kolkata at airasia official website. For accomodation, do a lot of compare and research in agoda,mmt,booking.com, expedia,tripadvisor and hotels.com. Believe me,its possible to do it in 30k +/- 3k if proper research is done beforehand ( i dont know if the visa charges are abolished for indians this year, heard it has been ).
Hope this helps. Happy exploring.