Word for "non-unpleasant"?

Since 'not unpleasant' is an understatement (litotes), an appropriate counterpart might be:

tolerable.

Consider

ameliorate

assuage

alleviate

palliate

mitigate

ease the pain

Mitch's suggestion of 'tolerable' is spot-on - particularly for the scenario you described.

Other adjectives that suggest "not unpleasant":

Here's a couple of ideas:

Gild the pill. Refers to the practice of coating bad-tasting pills with gold to make them easier to swallow.
Sugar-coat. Similar connotation as above, coating something that tastes bad with sugar to make the taste more palatable.
Beguile. To divert attention away from something unpleasant in order to make it pass more agreeably.
Pleasant. The word pleasant itself can be used as a verb.

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