I'd say that using Ruby with Sinatra is really easy to pick up - the documentation of Ruby itself is great and Sinatra is also documented really well, not that i think it's particularly complicated. Use an easy template-engine or stick with the default (erb) and (at least for the beginning) the sqlite-gem.

Of course, that was my impression given my prior knowledge, which of course influenced my start with that environment. But i found it to be great almost instantly.