Is it easy to extend a 60-day tourist visa to 90 days in Chiang Mai and do we need outbound tickets?

You'll need onward travel within 60 days to board your flight in the US

the above answers are 100% accurate. therefore i will speak only through experience. I have had at least 10 - 30 day extensions from CM immigration. Never had problem.

The 30 day extension to tourist visa is the easiest extension to get.

Immigration won't ask for proof of onward travel, but your airline might require you to have it before they let you board your flight to Thailand.

Depending on your passport country A extension of 30 days is almost certainly guaranteed to those countries allowed (in theory it could be denied but I’ve never heard of it being so). You do not need outbound tickets to get the extension. You’ll need a filled out TM7, a copy of your passport data page, a copy of your current entry stamp, a photo and 1,900 baht. Most immigration offices have copy/photo places on site. Forms available there. Sometimes you can find them online but some offices can be pedantic about their version only.