I think my family and I have been to Lagoon almost every year for the last thirteen years. We usually go in July or August and we have a great time. I know it's not quite as large as Disney, Universal or one of the other major theme parks but they have some pretty great rides and it's close to home.
It's also easy on the budget. If you live in Twin Falls, you could leave early in the morning, spend the entire day and return in the evening and not have to spend the night if you don't want to.
As many times as we've been to Lagoon, I have never visited in September. After my trip this weekend, I'm not sure that I will ever do it any other way. Here's why: