is it "enough" to just set them up as aliases
is it better to alias them and setup some kind of rewrite rules to make the content look like separate domains
No. If you do this you will have duplicate content issues as the same content can be retrieved from more then one URL. If you have multiple domains you will want to do a 301 redirect from the alternate domains to the primary domain. This will prevent duplicate content issues and if someone ever tries to use an alternate domain they will find the primary instead.
is it better to set them up as landing pages with slightly different content & then redirect to the main site?
No. Those pages will have no SEO value due to having little content and no links to them themselves. So links from those pages will be worthless. You're life will be much easier if you just redirect the alternate domains to the primary domain from the get go.