e-mail beforehand the archeological museum of Valletta and book a ticket to Hypogeum (or do it straight from Hypogeum). The entrance is limited, and with only three days it is wise to have it prebooked.

Explore Valletta incl. Archelogical museum and the main cathedral.

Take a funny colourful bus and go to Mdina, "the silent town". Cathedral + cathedral´s museum.

Explore Vittoriosa. It is even older than Valletta. Visit the inquisitor´s house.

The megalith temples, also others than Hypogeum.

Mosta church. Not so old but different from most of Malta´s churces.

Cannot think of anything else now.

Would you bother using a guide (got the name of Vince from another Fodor person in his report) but the 1/2 day is about 7 1/2 hours for a tour?Do you think that if you are staying in St. Juliens that you could actually just use the public transportation system and that would be enought? I appreciate your help!

Whether they need a guide depends on what your daughter is like: I was there 10 days, never felt the need for a guide, but had done a lot of research, read guidebooks, figured out what I wanted to see, knew the bus routes, and only needed help when I got a little lost. Virtually everyone speaks English there and they were enthusiastic about helping Americans (at least pre-Bush, they were).
If she's going to land in Malta with no idea of where she wants to go or how to do it, a guide might help her take full advantage of her short time.
Mdina's very atmospheric, no cars allowed, fascinating to walk around; there was a tiny cafe built into the walls/ramparts of the walled city that was a great place to go, have a cup of coffee, something light to eat, and look out over the island.
If she's into megalithic ruins, like Stonehenge but older, there are a number of places on the island, all pretty much accessible with a short walk from a bus stop.
The market day in Marsakklok (?sp) was a lot of fun; ate freshly caught fish in a sidewalk cafe, accompanied by what I thought was dyed orange juice but turned out to be juice of blood oranges (delicious!)
The Hypogeum was not open to the public when I was there but it's supposed to be fantastic.
If your daughter has access to email, then she must have internet research capabilities. She should google Malta and see what might interest her.
People here have raved about Gozo; to me, it was kind of uninteresting and didn't offer nearly as much as Malta itself did. There are numerous tours of Gozo offered, all requiring fairly long bus rides to the west coast of Malta to catch the ferry, then most of the day in a bus touring the island of Gozo, before returning by long bus ride to St. Julian's.

Thanks for sharing these with me as I am leaving tomorrow to meet her in Rome for her 21st birthday. I will print the info out to give to her.Anyone else have anything else to add before I leave?