91 Honda Civic overheated today (almost)

Hi,
I have a '91 Honda Civic, about 160K miles. Runs like a champ! Today ((an unusually warm day in SF, approx 70 degrees and sunny)) I was driving back from Oakland, (approx 20 mi drive) got stuck in traffic, and my temp. Gague rose up to "H" while idling. After we started moving a little, it went back down to normal. All was fine until I got a few miles from home and started up a long hill. Then it went back up to "H". Turning the heater on helped a little. Then as I got about 6 blocks from home (also severly uphill), it went back up to H. I made it home w/ temp still at H, but it never totally overheated. After a few hours, I started the car and let it idle for a little bit and all seems okay. I did notice at that point a little bit of green fluid had leaked out the bottom. Help! Any ideas on what the problem might be? And if this is an expensive repair?
Is it okay to drive short, hilly distances in cooler weather for a couple of days?
Any help would be so appreciated.