What: small aquarium in Mandalay Bay. They pack a lot into a small space. We saw sharks, Komodo dragon, piranhas, jellyfish, octopus. My daughter especially enjoyed the touchpool which had sting rays and other rays and we spent a bit of time there while she asked the worker a lot of questions.
Cost: my daughter earned a free ticket for herself through the Henderson library's summer reading program, so I just had to pay the discounted local adult price of $17.
Location: in the back of Mandalay Bay. You can avoid walking through the casino by parking in the back by the convention center.
It was fun since she had a free ticket, but it's small so we wouldn't come back unless she received a free ticket again in the future.