I'd suggest forwarding the domain to some sort of CPA or PPS based affiliate marketing site, but it sounds like they're not even getting enough type in traffic for that.
If you're going to be making minisites on them, you'll probably be able to get some decent revenue...
For a start, you'll be able to get them listed and crank in a bit of SEO if you wanted.
That said, if you've got them pointed to a decent CPA or PPS based site, say you're getting $15 per action/sale, then the site pays for itself if it's only making one conversion every 2 years... And if you've got some semi-decent domains, you should be able to do better than that.