Drones - What makes performances great in addition to correct technique, styles, rhythm, is the ability of the performer to play with great intonation. The use of a "drone" (the constant sounding of only one pitch) while practicing specific selections or ad lib is a valuable tool to better the student's ability to hear notes in tune. The Marsha Sloan Cello Drones (which are great for use while practicing any instrument) are available for your use via Youtube. Here is the link to the specific playlist. www.youtube.com/results?search_query=marsha+sloan+drones
Metronomes - In the same way having a great rhythmic sense is invaluable to great performance. There are many good metronome apps for all of your devices. But having your own metronome is the standard for use during practice time. Check this link for more information on metronomes for sale. www.amazon.com/Best-Sellers-Musical-Instruments-Metronomes/zgbs/musical-instruments/11965901