At one of the late games in 2004, we actually ran non-stop (switching between PlayStation2 and a DVD player) for 8 hours on one boat battery. No joke.

An average boat battery costs between $39-$69 (or pull one from your boat for the day). A 400 watt inverter ranges around $30-$40. Both items are easily accessible at stores such as NAPA or Advanced Auto Parts. So, for anywhere from $69-$109, you can be completely set up to run up to 8 hours worth of stuff. Just be sure to double-check and total the wattage of your TV, game console, and perhaps a DVD player. Make sure this sum is less than the normal usage rate (not the peak usage rate!) of the inverter you buy

As a final tip, you might want to think about bringing 2 batteries in case one dies, or wasn't fully charged. We do, and it's come in handy a few times... including once when we had to jump start our car when it died from playing the radio too long :)