In a nutshell

Sennheiser CX 300 or Sennheiser IE 4 (or other brands) are light and small in-ear headphones provided with different ear adapters for perfect adaptation all in a small convinient protection pocket. They perfectly adapt monitoring systems like Sennheisers IEM or Lectrosonics IFB.

Earbuds like Sennheiser MX 270 (or similar) are also available.