Wayne wrote:
I'm drawing a blank on a good/easy to build antenna.

It of course depends on the frequency you are going to use.
WiFi antennas with gain (both directional and omni) are available
off-the-chinese-shelf (i.e. at prices you are not going to beat), but
of course you can always use one of the wellknown technologies to
make one yourself. i.e. a yagi, loop, doublequad, dish, etc.

But first find out on what frequency you are!