Brand-able and memorable domains are only worth what someone wants to pay for them, that's if you were selling them. And that's worth about as much as your letter only domains. I assume you mean a few random letters. Unless a multi million dollar company finds them you wont get many big offers.
From an SEO perspective. Ranking 1 domain is difficult enough, and time consuming to be done right. Build content, regularly update the site with new content. Get incoming links etc. When you do that you can rank any domain for any keyword regardless of the keywords in the actual domain name itself.
I would suggest putting them on to https://flippa.com/ or sedo. Though you'll notice the domains which are selling for high prices have a few things going for them. Verifiable traffic usually they'll share access to Analytics to serious buyers. Steady income, again with proof or generated revenue. I'm seen sites sell for over $1,000,000.00 on there.
If you aren't looking to sell and want to generate income off the sites you'll really need to spend a time getting them ranking. Once in the top 10 for keywords which are producing some traffic you can throw ads on the site and affiliate offers.
Decide what category each domain would fit into. At least the ones which you say are brand-able. Now Google related terms as well as run a search on Amazon. See what products and services are out there, what kind of affiliate programs and decide which website in your portfolio stands to generate the most income for you then spend time on that one.