if you have just one day -

1- st. peter's (i like the museum a LOT, and I go every time I am in town, but it can take a long time if you aren't willing to skip bits, so with one day, I agree with the other poster - skip it)

2- coliseum, titus' arch, forum, and up that road towards the vittorio emanuelle monument and square. then up the main road toward piazza navona, the pantheon and the travi fountain, ending up by the spanish stairs (for all of these, you need to go off the main road in various directions. I always forget this street's name). there are lots of really interesting things to see all around this area - fountains churches, etc. worth just wandering, by all means.

3- don't forget to get amazing gelato at Giolotti (near the pantheon) or san crispino