Well, Old snowy is basically right on the PCT route, so that’s not really going to take much time. You could climb Mt. Curtis-Gilbert, which is the highest point in the Goat Rocks (essentially, it’s the high point on the other side of the ridge). It’s a bit of a scramble (especially near the top), but can be done from the PCT side. but, really, the PCT goes right through the best of the Goat Rocks, so there’s not a whole lot extra you can do for a whole day that’ll be much nicer than the PCT. (Oh, I guess one thing would be to go down to Goat Lake and such for a change of view…)
Another option is to climb Mt. Adams. There’s actually more snow on the south route which can be good or bad depending on what you’re looking for. The north side is more of a ridge, but it’s pretty crumbly volanic scree… I didn’t find anything too dangerous there (unless you get off-route), but it’s a bit of a slog. Even if you don’t summit, heading up above the meadows high on the north side of Mt. Adams (name escapes me at the moment) would be a good side adventure.
Good luck out there!
jonathan