"Not too bad" is a colloquial expression that essentially means "okay" or "acceptable". While it can imply a lack of enthusiasm or excitement, it also conveys a sense of moderate satisfaction or contentment. Alternatives to this phrase include "fairly good", "alright", "tolerable", "not terrible", "passable", "acceptable", "moderately pleasing", "satisfactory", "decent", "respectable", and "so-so". Each of these phrases conveys a similar level of satisfaction or approval, while offering slightly different nuances and shades of meaning. Ultimately, the right synonym will depend on the context and the speaker's intention.