Ellen has visited a couple of other people's worlds because they've basically indicated she needs a vacation, but she doesn't really grasp the concept and isn't very good at it. She might be able to handle a vacation to a world that isn't Earth, since it'd neither be something she could see as recon/research (Mr. Mills' Washington, DC) nor a horrifying 'this will be dead when I get back to my world' realization moment (Steve Rogers' New York City), but even that's a bit iffy.