Definitely a good start! Here's a couple suggestions: it might be a bit of overkill with using 一定 in conjunction with 確定. 期盼 might sound a bit formal for the arrival of a friend. Here's a colloquial rendering:
Colloquially, I think 确定 should be used instead of 一定. It's not incorrect to use 一定, but it sounds too forceful. I also agree with it being a bit of overkill using them together. It doesn't sound right at all, and native Chinese speaker would not normally use them like that. This is your original: "因为我很确定我们一定会玩得开心." 很 is redundant because if you are already certain, you cannot be EVEN MORE certain, instead the 很 should be play before 开心. I would write/say it as, "因为我确定我们会玩得很开心.