"Seeing the Grand Canyon was certainly____(an /some) experience." Is experience countable or uncountable? Should I use some or an?
In some instances, it's countable and in other uncountable, depending on its meaning. According to the Longman dictionary if it means "something that happens to you or something you do especially when this has an effect on what you feel or think", it's countable.Seeing the grand Canyon, of course, has a great effect on everybody, so in this case it's countable and you should say: "Seeing the Grand Canyon was certainly an experience". More examples:
Ballooning is quite an experience.
I have a lot of experiences from trip to Africa.
Seeing the Grand Canyon was a really fantastic experience.
But if it means "knowledge or skill that you gain from doing a job or activity", it's uncountable.
And yet one will hear far more native speakers saying
Our visit to the Grand Canyon was some experience!
That was some experience, when we got drunk and went over Niagara Falls in a barrel!
Some experience connotes that the experience was memorable in some positive way. An experience can connote that it was quite an ordeal. As can some experience, except that the pleasure is in telling the tale of the ordeal.
So, depending upon how much the barrel riders enjoyed the falls, we might also have:
That was an experience, when we got drunk and went over Niagara Falls in a barrel!