Shield Spell is a "Reaction" Spell that can be cast out of your turn and some cantrips are bonus action spells as well as some higher level spells but I see these Higher level spells require a Weapon Attack "Action" first
or are Healing Word spells
Your "Turn" is casting a "Bonus Action Spell" and a Cantrip "Action"
from SILVER DEFENDER:
Almost. If you cast a spell that takes a bonus action to cast, you may only cast a cantrip in addition. PHB page 202.
but what you are asking is "Can you cast a "Bonus Action" Spell as a "Action"", why would you waste the "Action" when you can use the extra "bonus action", and per the book, no you can not cast the "bonus Action" as an "Action", but asked the DM, DM might allow it
Book: You must use a bonus action on you turn to cast the spell, provided that you haven't already taken a bonus action.
if the spell has "Bonus Action" and "Action" in the spell description, some Action spells can be cast as rituals, but is written that way, I did not see a "Bonus Action" spell listed as a "Action" to.
Our Group was doing this "Cure Wounds Spell" "Action" and "Healing Word" "Bonus Action", but we did it wrong