If you want to fire up your app and play around via the command line the easiest way I found is to use irb.
Just run with irb with -r. The bundle exec is optional.
bundle exec irb -r app.rb
if you don't mind another dependency, you can add racksh to your gemfile. I've found it extremely useful and it allows you to use a .rackshrc file for configuration.
Thanks for the tip Kormie!
If the command can't find your app.rb file, make sure it's in your LOAD_PATH:
bundle exec irb -I. -r app.rb