what would have excited Montmorency to go for a river trip ?

There was nothing about the river trip that would have excited Montmorency.

The only one who was not struck with the suggestion was Montmorency. He never did care for the river, did Montmorency.

“It’s all very well for you fellows,” he says; “you like it, but I don’t. There’s nothing for me to do. Scenery is not in my line, and I don’t smoke. If I see a rat, you won’t stop; and if I go to sleep, you get fooling about with the boat, and slop me overboard. If you ask me, I call the whole thing bally foolishness.”

This quote can be found in Chapter One of the text. If your question pertained to a different chapter, you will need to provide that information.