Antonyms for the term "spice up" suggest ways to tone down or make something less interesting or exciting. The opposites of adding flavor or excitement to something could be blandness, soberness, plainness, or dullness. To make something less interesting, you could use verbs such as dull, mute, quiet, or hush. Another opposite of "spice up" is to make something straightforward or simple. This could be described using words such as simplify, condense, abbreviate, or compress. All of these antonyms for the term "spice up" can help you create a more thoughtful and thorough approach to communication or presenting information.