Golos activated ability and adventure cards
You can do both.


  715.3. As a player casts an adventurer card, the player chooses whether they cast the card normally or as an Adventure.


Playing a spell is the same as casting it. You're right that the adventure mechanic does mention alternative characteristics, but this is different from alternative costs.


  118.9. Some spells have alternative costs. An alternative cost is a cost listed in a spell’s text, or applied to it from another effect, that its controller may pay rather than paying the spell’s mana cost. Alternative costs are usually phrased, “You may [action] rather than pay [this object’s] mana cost,” or “You may cast [this object] without paying its mana cost.” Note that some alternative costs are listed in keywords; see rule 702.