If you are planning to spend your layover in the airport itself you don’t need a transit visa. However if you are planning to leave the airport premises for a short period you do need a transit visa. For application of transit visa, you need passport, your visa, proof of your return ticket.
Yes you need a visa to enter Laos. Since you are eligible for VOA which is valid for 30 days, if you prefer you can go for a visa online or opt for VOA. It is totally your decision.
If you have not visited Bangkok before then you have a golden opportunity to visit Bangkok in very very compact budget. So take very small quantity of goods with you and start exploring Bangkok. And YES you will need the transit visa when you leave the airport. For application of transit visa, you need passport, your visa, proof of your return ticket.
Listing down some options you can do in your layover - 1. Relax at Suvarnabhumi Airport 2. Visit Khao San Road 3. Visit Wat Pho Temple
4. Visit Erawan Shrine
5. Taste different Thai delicacies
6. Visiting Grand Palace
7. Siam Paragon mall
If you want to Explore Bangkok means outside Airport then you should need Transit Visa otherwise if you spend your time inside airport then no need for transit Visa