Neutral:
* Common
* Usual
* Typical
* Standard
* Average
* Normal
* Regular
* Everyday
Negative:
* Mundane
* Banal
* Unremarkable
* Uninteresting
* Dull
* Tiresome
* Uninspired
Positive:
* Simple
* Basic
* Straightforward
* Unpretentious
The best synonym will depend on the specific situation. For example, if you are talking about a boring day, you might use "mundane" or "tiresome." If you are talking about a simple meal, you might use "basic" or "unpretentious."