Almost :) The lake is frozen, let's go skating! You may prefer this..

I feel that you have written perfectly correct English. It does not need to be changed (except to put a period after the second sentence). Craig Woodhouse is suggesting an improvement in style. What you wrote was correct. He has made it better and more idiomatic. A sentence contains a single thought. If the "The lake is frozen" and "Let's go skating" are so closely connected that they feel like a single thought, then it is proper to combine them. You can do that by using the comma, and not capitalizing the word "let's." Since there is a feeling of excitement or emotion, it is proper to put an exclamation point at the end.