Sand Timer (Best Sand Timer by Serhii Simkovskyi https://appsto.re/us/glW6D.i) - eye catching and audio pleasant hour glass timer. Set the time, press start and then sweet sounds will let you know "times up".
Too Noisy (Too Noisy Free by Walsall Academy https://appsto.re/us/aTdS8.i) The students will get a kick out of this one. The app measures the level of noise in the room. If all is well, there's a smiley face set to a pleasant background. But once levels rise to unacceptable levels, the smiley face goes away and the emoticon puts its fingers in its ears and the background changes to unhappy features! Going Pro offers more "whistles and bells". This one's fun!
When searching the App Store, I found plenty of apps that pretty much do the same thing, but these seemed to work best with our younger friends in Elementary School!