In the song, the ding-a-ling is (supposedly) a bell, so that's an onomatopoeia since bells make that sound (the euphemistic meaning in the song is something that dangles like a bell). Bling (shortened from bling-bling) is also more or less an onomatopoeia - the cartoon sound of light glistening off gold/gems or the sound of lots of jewelery clinking together.

thingamajig, watchamacallit, gizmo, etc. are often called placeholder names. A rather extensive list in multiple languages can be found here : http://en.wikipedia.org/wiki/Placeholder_name

Some doo-wop lyrics can be placeholders (Who put the bomp in the bomp-shu-bomp-shu-bomp?) because they are taking the place of other words, but some are merely exclamations or a possibly form of scatting (Be-bomp-a-loo-bop, a-whamp-bam-boom) or maybe early beatboxing.