Hello Ash47b,
if I got it right, you are going to stay in Tuscany for a week, correct? 2 days in Florence and the rest in Siena travelling around the region..
Well, since you are in Siena,
Chianti is definitively worth a visit and easy to do! I won't suggest you a town in particular, because the best thing to do in Chianti is just admire the landscape, taste some excellent Chianti wine and enjoy the atmosphere. And you can do it everywhere!
About the seaside... I would suggest you to head to Monte Argentario (1 hour and half from Siena), it offers transparent water and incredible landscape (I was there last week, it was fantastic

). The beach of Follonica is also really beautiful but if I had to choose I would absolutely go for Argentario!
Then, if you still got time to spend exploring Tuscany I suggest you to see:
- Montepulciano (1 hour from Siena), a lovely medieval village, which offers breathtaking landscapes and panoramas, close to other beautiful towns in
Valdorcia: Pienza (the town of a prestigious sheep milk cheese) and San Quirico d'Orcia. They are very close to each other so you can visit 3 lovely places in one day!
-
Arezzo (1 hour from Siena) another Tuscan pearl not to miss, plus Volterra and San Gimignano!
Hope it will be useful for your holiday!
Sara