She's not getting any success ramming through the walls being a relatively lightweight balloon and all. Compared to a flesh and blood creature, that is. She lacks the heft to shatter the wall, but still persists, bouncing with loud, hollow reverberations harmlessly against the wall.
She gives up after a while, sitting on the floor frustrated with her inability to escape. Then she finally thinks a bit. She's a balloon, right? No internal organs, lightweight... if she wants out, it doesn't matter that she's bigger than the door.
She plunks over to the front door and presses her right "hand" into it, the soft balloony texture forming around the knob. She twists it to the left a bit and voila, the door opens.
It's not easy, but first she manages to press her head through the door, pushing air pressure to her rear quarters. She continues pressing her way through, uncomfortably so, but ever so slowly progresses out the door.
Except for her rear end, which is quite simply stuck. She struggles, squirming and wriggling her way out, but isn't exactly succeeding. Panic begins to set in again, and she wriggles more furiously, thrashing about with loud squeaking sounds.
And then, finally, the force generated by her thrashing is enough to burst her loose, and she flies out of the door, rocketing out with a speed that hippos who aren't balloons who were once humans don't normally attain.
It's a thrill, a rush, the wind against her rubber exterior, the sight of the ground rushing below her... and she's got no signs of going down, even when she slows down, floating idly in the air, she isn't descending. She begins to wonder if she's filled with helium or something, and then stops thinking about it. Floating about... so peaceful, so relaxing. No need to worry about anything.