Hi Heidi! I think “experience” is correct, because “survival training course” is the name of a course, in which multiple activities can be found. It is a collection of activities. “A great activity” would be a single exercise so it would be one activity, while ”a great experience” can last for example multiple days. If there is only 1 exercise about how to make a fire, then it could be “a great activity” and/or “a great experience”. But then it wouldn’t be called a course. If the training consists of multiple things, for example an exercise about making fire, an exercise about making drinking water and an exercise about gathering food, then you would say “the survival training course was a great experience”. So the word experience can be used for a combination of things: “studying in university was a great experience” While activity is used for a single action: “the teacher started his lesson with a fun activity” Hope this makes sense?

They are both okay. Nayan's description of the word "activity" is technically correct, but most native English speakers would not be so strict about it. It would be quite common to hear the word "activity" used to describe something bigger that contained many smaller activities. So both are okay, but there is a subtle difference in emphasis. If you say it was a "great experience", you are emphasising it personal impact on you. If you say it was a "great activity", you are probably making a more objective statement about the value of the activity in achieving certain goals.