I'm going to say the obligitory "Get a BMX"



BMX Bikes are:
- Cheaper to purchase
- Easier to find
- More durable
- Cheaper to maintain
- Have a much better geometry for tricks
- MUCH easier for learning tricks. MUCH easier.
- Will fit inside your car (no need for bike racks)

Seriously, $500 new and $2-300 used for a quality BMX bike.


EDIT: Plus the catalog of tricks for BMX is HUGE. Not so many for fixed gears.