Yes would love to know 2 or 3 best options for natural site and man made attractions in Thailand. And best places to which covers almost most famous views there !
Coming to man made attractions, Wat Pho (sleeping Buddha) is a must see and so is the Big Buddha in Phuket. A special mention for man made attraction would be the New Year's fireworks at Bangkok or lantern festival in Chiang Mai. Also, go for the Full Moon Party at Koh Phangan if you can.
The third favorite would be the rice paddies in Chiang Mai. Being a scuba diver I would like to mention the underwater beauty that Thailand has to offer. Even though Philippines is my favorite destination to dive in, I would suggest you go diving in Koh Tao as well.
Do take a look at my blog for more reference:
https://www.gonimad.com/blog/adventures-of-a-solo-indian-traveler-1 https://www.gonimad.com/blog/adventures-of-a-solo-indian-traveler-2
I would suggest make the trip a week long since you are willing to take 4 - 5 days leave from work you might as well include the weekends. Stay in a hostel and take public transport whenever you get a chance.
Well for starters you can take a cheap flight (say Indigo/Thai Lion). If you want to cover the touristy side of Thailand you can land in BKK/Phuket and make a trip to Pattaya.
April should be a good time as the weather is perfect for a beach holiday and it hasn't yet started raining in Thailand. Also, many places will be less crowded as the holiday season is over. But do be prepared to face a lot of heat and humidity.
Thailand would be opening up the ports from July 1st hopefully but for Indians to travel to Thailand there might be some rules and regulations as the Covid-19 scene is getting bad there. Thailand hasn't reported cases for a while so they soon might open up bars and clubs. Finally! 😁