If you're frequently switching between projects, you might be annoyed by RubyMines behavior of opening the last project on startup.
After all loading a project takes a few seconds as files are scanned and the RubyMine index is rebuilt. If you switch to another project after startup this time is doubled.
A saner default might be to open RubyMine with a dialog to choose the project to open.
This can be set under Appearance & Behavior > System Settings > Project > Reopen projects on startup.
Switching to other projects
A helpful dialog for switching between projects is Manage Projects.... This provides you with a list of recent projects that you can switch to (which you can filter).
Best add a shortcut like Ctrl + Shift + P to it (File > Open Recent > Manage Projects... in the settings).
