What tense should I use, present or past?

They're both correct, but have different contexts, and reference to time.

"I think this is an exciting activity'. This could be an activity you regularly do. You have done it before, you could be doing it at the moment, and you are likely to do it in the future. That's why a present simple stative verb is used to describe something that is relatively constant, repeated and unchanging.

"I think this was an exciting activity." You might have only done the activity once. It is now finished. You are less likely to do it again. :)