Siena is at least a 6 hour drive from the Amalfi coast, so hard to see how you could do "a day trip" to/from there. Here's the thing, you could spend a month on the Amalfi coast and not run out of things to do. "How much time" always seems like the big question, and no shortage of opinions from people to say "oh this or that place is only worth 27 minutes."
You could spend a day or a week in Positano (for example): it all depends on what your goals are, what you want to see, and how fast do you want to see it? Are you going to sprint from place to place, hardly getting your feet dusty before you run off to the next "attraction?" Personally, I'm not a fan of that kind of travel. However, if all you want to do is hit the highlights and move on, you could still spend a week in Amalfi coast and not see nearly a quarter of everything there is to see/do.
So, personally, I would start off with my usual advice (that was given to me): always travel as if you'll definitely be back somewhere - you can't, won't and don't want to see Italy (all of it or major part of it) in a week. So decide what your goals are, and satisfy them.
If you are centered in Amalfi, then you are talking almost 3 days to see Siena - a day to get up there including checking out of where you are staying, driving (or worse, taking public transportation which turns 6 hours into 10), arriving, etc., a full day to see Siena (and if you are going to go back in the evening (unrealistic unless you're driving and able to cop some No-Doz), you have to find somewhere to leave your luggage, etc. or stay another night), and then 6 hours or more to get back. Sounds like a waste of three perfectly good days you could use to explore Amalfi (town), Positano, Ravello, Salerno, Paestum perhaps: that's 4 days right there, a day for Capri (5), a trip or 2 to the beach, some hiking, and your 7 days is done (and overdone). It also takes a day or two to get the lay of the land somewhere - get your bearings kind of thing.
So anyway, as I was saying, no one can tell you how much time a place is worth except you, once you take into account all of your goals and what you want to see, and what kind of traveler you are. Finally, you can never do anything as quickly as you think - time always runs more fluidly when travelling, and what you thought you could do in 2 hours probably will take 4, 4/7, 7/all day.
Anyway, whatever you decide, have fun, and travel safely!