The easiest way to get a new 45-day visa-exempt entry (you did not enter on a tourist visa, it was visa exempt) is to do a border bounce. If you're still in Chiang Mai, according to a post yesterday from
@Tod ********
, Chiang Khong is out as an option.
Presuming you haven't got a long history of being in and out of Thailand, fly to another country, spend a bit of time if you fancy and then come back and you'll get a new 45 visa-exempt entry.