Your little sister has come up with some great ideas. Since this is fourth grade essay I think all she needs to do is elaborate of the things she chose. For example, she likes to tan. She should discuss how this makes her feel. She must like the warm sun's rays, perhaps while she is building sand castles on the beach. She also likes to have picnics. Many times when people have a picnic they go to a state park and go hiking. Does she like to plant and pick flowers? She could discuss this too.
There are also many more visual details that describe summer besides sparkling water and blue skies. How about the green grass and the vibrant flowers. During summer there is also the hum of motorcycles going down the road.
I think she did a great job on her details. All she needs to do is run with them a bit more.