The following Icelandair destinations (out of Reykjavik Keflavik) will be among the first to feature the 737 MAX, per @airlineroute:
Berlin as of March 19, 2018
Paris Orly as of May 1, 2018
Birmingham as of May 2, 2018
Hamburg as of May 3, 2018
Billund as of May 7, 2018
Cleveland as of May 16, 2018
Philadelphia as of May 22, 2018
Amsterdam as of May 24, 2018
Stockholm as of June 1, 2018
Halifax as of June 2, 2018
http://onemileatatime.boardingarea.com/2017/09/13/icelandair-737-routes/