I actually thought about running some geothermal plumbing in my yard while installing the greenhouse. The problem with that is 1/2 my yard, and 1/2 lazyness. I can blame part of it on my yard because I have a thin one foot layer of sand, and then nothing but wet clay for as far as I have ever dug.
I cannot imagine the trouble I would have digging a 3-5 foot deep trench in clay to lay the pipe. I DO think geothermal is a good idea. I would like to bury some pipe if I ever build a larger greenhouse on a different piece of land.
For this little greenhouse, I have looked into chillers and small window ac units. A chiller would be fantastic....for $300-$500
But, being a cheap DIY guy, I will probably just go with a $70-$80 el-cheapo window unit from Wal Mart. I bet an energy efficient 5000 BTU model would be able to take the edge off those really hot days using about 500 Watts of power.