Muptiple domain seo question

I have an established website running under a domain name (for the purpose of this example)

Some time ago I also purchased the domains, but I have kept them parked.

How best could I utilise these parked domains to promote the original/established site?

Should I try to build up each of these domains as separate websites and link to the main one? Or should I just redirect/forward visitors straight to the established site?

Basically I just think that their potential is being wasted by just keeping them parked.

Thank you for any advice.

I would weigh the amount of effort it takes to develop those parked domains against the expected benefit they’ll bring. My own opinion is that you’d probably see greater gains from putting that effort into your existing site.