Hey there is a casual greeting that can be replaced with other informal expressions. One synonym would be 'hello there', which conveys a similar tone and warmth. 'Hi there' is another option that is commonly used in informal contexts. Similarly, 'yo' can be used to greet someone in a laid-back and friendly manner that showcases a more casual vibe. 'What's up', 'howdy', and 'greetings' are a bit more colloquial and can be used in certain situations. Other variations that can be used include 'hey', 'hello', and 'hi'. Regardless of which word you choose, it's important to consider the context and your relationship with the person before addressing them.