Yo-yos - Everyone has had one of these at some point in there life. A trip down memory lane combined with learning something new. Very portable. The ultimate pocket sized workshop.

Diabolo - Otherwise known as the Chinese yo-yo. Easier to pick up than juggling but just as challenging in the long run. Requires less space than you might think (and no, I didn't spell it wrong).

Cigar Boxes - A fun and surprisingly aerobic juggling prop made famous by W.C. Fields. The basic tricks are very achievable and can be learnt in minutes.
For a quick demonstration, click here

Party Tricks and Miscellaneous Skills - A crash course in skilfully showing off. How to spin a ball on your finger, shuffle a deck of cards with one hand, balance things on your nose and many other party tricks and miscellaneous skills with everyday objects.