Your issue isn't the visa to Thailand. Posters above have made this clear.

Your issue is ENSURING THAT YOU ARE NOT DENIED BOARDING on your initial flight from (USA??) to BKK. I would rate a 90% chance of being denied boarding without EITHER a flight out within 30 days of arrival, or a visa.

A Thai Tourist Visa would solve that problem. So would an air ticket out.

By the way, a HKT-SIN can be as little as $50. A visa would be more, including postage. Better to buy a ticket that you would use, don't you think?

As for coming and going to/from Thailand, you have a few choices.
Think about which is best depending on HOW MANY TIMES you will be entering the country And HOW MANY DAYS for each entry.

Airport entry to Thailand with no visa = 30 days. (extendable by 7 days at Thai Immigration office 1900THB)

Border entry to Thailand with no visa = 15 days. (extendable by 7 days at Thai Immigration office 1900THB)

You can apply for a Single Entry, a Double Entry, or Triple Entry tourist visa outside of Thailand (at either airport or border). Each entry = 60 days, which EXPIRES the moment you leave Thailand. (Each entry extendable by 30 days at Thai Immigration office 1900THB)

If you enter on a tourist visa, you can keep your 60 day stamp active by applying for a re-entry permit (this applies with any active VISA entry). Single re-entry or Multiple re-entry stamps are available from a Thai Immigration Office within Thailand (price should be 1900THB)

Still sounds like a throw away ticket may be the cheapest. If, and only if you need more than 14 days on your subsequent entries, then a multiple entry tourist visa will be a good option.

You'll need visas for the other countries you have listed, Laos, Vietnam, Cambodia too. Vietnam required some type of pre-approved paperwork, if I remember correctly. Laos and Cambodia visas can be purchased at point of entry.