"It is a sunny day today. Let's go out and play!"
Here, 吧 is used to express interjection of the sentence. If you want to literally translate the given sentence, you can say 今天是晴天，讓我們出去玩。
The last part is generic, which can mean that some of the subjects go out and play. If you want to include everyone in the sentence, the sentence is 今天是晴天，讓我們一起出去玩。, where 一起 emphasizes that 我們 do the job.