Can I re-enter Thailand on a visa exempt basis after leaving with a single entry tourist visa?

You can leave Thailand on your single entry tourist visa and return as a visa exempt traveler. After you exit Thailand, you will receive a visa exemption stamp for 30 days upon re-entry, unless you return after October 1st, in which case you'll receive 45 days. Your existing visa will not impact your entry, as it is single-entry and only valid for one initial entry.