Sinatra looks very straight-forward. Checking out the repo on GitHub now, and it looks great! I will probably do some sort of token-based auth. The app will eventually integrate with Ember.js.
i don't know if this will be too confusing for you if you're new to sinatra, but the new version of Kibana (which is basically an interface to elasticsearch) uses sinatra, angular and grunt. Basically there are some grunt tasks to package it up into jruby so the final app runs wherever java is installed.
https://github.com/elasticsearch/kibana