This is a linguistics forum, and we should expose non-native foreros to as many similar phrases as possible - it's a step on the path to fluency.
Here's a google with many nice ones, of which these are outstanding:
Happy as a clam (very common in AE)
Happy as a lark
Happy as a pig in feces
Happy as a dog with two penises
Happy as a flea in a doghouse
The forum is PG-13, so some of the words have been modified from the original.
I guess some of these colloquialisms get truncated over time across oceans. I have heard the whole phrase, it resonates, but I think just the first half was a favorite of my father. A place for wonderful little metaphors of the type is older people in the southern US. The language there can be pure poetry.