Informal: This is the most common and general synonym.
Casual: This implies a relaxed and comfortable atmosphere.
Unceremonious: This emphasizes the lack of traditional rituals or procedures.
Spontaneous: This suggests something happening without planning or preparation.
Free-and-easy: This describes a situation where there are few rules or restrictions.
Unpretentious: This implies simplicity and lack of ostentation.
Relaxed: This describes a comfortable and informal atmosphere.
Unconventional: This emphasizes a departure from traditional norms or customs.
Informal: This is a more general term for a lack of formality.
Direct: This can imply a straightforward and honest approach, without unnecessary politeness.
Simple: This suggests a lack of unnecessary complications or complexities.
The best word to use will depend on the specific situation and the nuance you want to convey.