I’m an Indian citizen with a US visa traveling from US to Singapore on United Airlines, and after 8 hours have another flight on Air Asia to Thailand. I’m wondering if I can do free tours from Changi airport? If yes then do I need to collect my boarding pass first for Thailand to show the proof of connecting flight?
The tours, which were suspended because of Covid, are back as of April 3rd. https://www.changiairport.com/en/airport-guide/facilities-and-services/free-singapore-tour.html has the details. (Note: the Heritage tour doesn't start until June.) All four tours look really good, and match up quite well with things I'd take a visitor to if we only had a few hours.
You need a layover of at least 5.5 hours and it has to line up with a tour you want to take. There don't appear to be restrictions like "your itinerary must be all one ticket". You do need a valid entry visa (since you are going to enter the country, after all) but that is not likely to be difficult. There's a detailed FAQ with links at the Changi page above.