For 300 nm from Montreal, try Sudbury, Muskoka or the St. Laurence North or South Shore towards Baie-Comeau or Rimouski (mind you, it's almost cheating to just follow the river). Following the St. Laurence and stopping at Quebec to gawk at the old town and/or overnighting at Ile d'Orleans should satisfy the tourist instincts.
The folks where you rent a plane from will certainly be able to suggest some good 300 nm XCs.