I've done two day trips (guided tours) out of Bangkok:
- Ayutthaya
- Hellfire Pass & Tiger Temple
Both were great, but ultimately, it depends on how many temples you've already seen. If you'd like to see some of the most historic sites & temples in Thailand, then visiting Ayutthaya is a great move. I'd recommend doing it with a tour, so you can get all of the information and actually know what it is that you are seeing.
The Tiger Temple is several hours outside of Bangkok. On the tour we did, you stop on the way at the some historical sites, a floating market, as well as see the bridge over the River Kwai and finally end up at the Tiger Temple (almost at the border with Myanmar). If you've never heard of the Tiger Temple, it's basically part Buddhist monk living space, part "animal sanctuary" (I put that in quotes, because there's some controversy about how the animals are treated... no clue what's true). You can pet tigers, walk one, get lots of pictures taken with them, feed and play with tiny baby tigers, and play with bigger tigers. It's definitely an interesting experience, although some similar places exist elsewhere in Asia.