Without good posture, it's impossible to reach one's full musical potential! Here are some fun ways to help with posture. (All of these come in multiple fun colors!)
Violin Bow Force (For Violin and Beginning Violas): https://www.sharmusic.com/Accessories/Teaching-Aids/Bow-Force---Bow-Training-Aid.axd
Violin and Viola Shoulder Rests and Chin Pads (recommended brands are Everest and Kun) https://www.sharmusic.com/Accessories/Shoulder-Rests/?keywords=&pageLength=36&cid=1320
Eva Pirazzi $$$ (for Violin) https://www.sharmusic.com/shop.axd/StringsWorksheet#fq=ATR_Brand:Evah%5c+Pirazzi%5c+%5c(Pirastro%5c)&fq=ATR_Instrument:Violin+OR+ATR_Instrument:Viola+OR+ATR_Instrument:Cello+OR+ATR_Instrument:Bass
A bow can change how easily a student can play, and effect the quality of their tone. Here is a fun upgrade for a student bow: Presto Spark - gold, silver, blue, red https://www.sharmusic.com/Bows/Violin/Beginning-0--125/Presto-Spark-Carbon-Fiber-Violin-Bow.axd
Fun Cases: https://www.sharmusic.com/Cases-Bags/Violin/SL-Super-Light-Compact-Violin-Case.axd#sthash.1g28a4Im.dpbs
Tuners: I recommend D'Addario micro tuner for violin and viola because it can stay on the instrument and be used during playing as well. The Shar neon clip-on tuner is great for cellos and basses. https://www.sharmusic.com/Accessories/Metronomes-Tuners/