A going away gift from Julia and Pat
The towel references is from Hitchhiker's Guide to the Galaxy
"... a towel has immense psychological value. For some reason, if a strag (nonhitchhiker) discovers that a hitchhiker has his towel with him, he will automatically assume that he is also in possession of a toothbrush, washcloth, soap, tin of biscuits, flask, compass, map, ball of string, gnat spray, wet-weather gear, space suit etc., etc. Furthermore, the strag will then happily lend the hitchhiker any of these or a dozen other items that the hitchhiker might accidentally have "lost". What the strag will think is that any man who can hitch the length and breadth of the galaxy, rough it, slum it, struggle against terrible odds, win through, and still knows where his towel is, is clearly a man to be reckoned with."
The "DON'T PANIC" (always in caps) reference is also from the Hitchhiker's Guide series.