It took a fair amount of trial and error to figure out my TARDIS balloon design. I'd still like to do a full-sized one but I'll need a good reason and some space--my house just isn't big enough.
Here's the one I finally ended up with:

This one I like, I keep.
It did my nerd heart good to see the family playing with the Doctor Who balloons I've had laying around for the last couple of days:

Cordelia in the TARDIS junkyard

My son's stuffed cat doing battle with a dalek.

Not a great TARDIS but an excellent hat. Note the vanquished dalek in the background.