Let's look at your sentence and make it better step-by-step!
Your original sentence:
"A day at the beach is great fun! You can lie in the sun and build sandcastles. or you can Swim with dolphins and sea turtles. But watch out for the scary sharks and jellyfish that can sting!"
Step 1: Fix the sentence after the first period. Instead of starting with a lowercase or capital 'or', connect the ideas properly. We can combine the sentences with 'or' or make them separate sentences.
Step 2: The word "Swim" should start with a lowercase letter unless it's at the beginning of a sentence.
Step 3: Make sure the sentences flow correctly.
Here is the corrected version:
"A day at the beach is great fun! You can lie in the sun and build sandcastles, or you can swim with dolphins and sea turtles. But watch out for the scary sharks and jellyfish that can sting!"
See? Now, the sentences are connected properly and the words are in the right case.