Hi Veronica,
Padang Bai is a good base for diving trips, if that is what you enjoy. If not, I'd cut the time there (or skip entirely, or leave as a day trip), and use those days somewhere else--e.g., day trip to Tirta Ganga/Ujung, or a night in Amed (again, good for diving). The hike up Agung is a bit strenuous, and requires a very early morning start--ideally with a guide. Another Agung option is to drive up to the temple on the mountainside; early mornings have the best views depending on cloud cover. Note that climbing Agung is not allowed when major events are going on at Besakih--and these are often in April. If you like rafting, Ubud is a good place to do this. Some outfits offer mid-afternoon trips which omit the usual lousy lunch, and are a lot cheaper. Kuta knick-knack shopping is cheaper than Ubud (and Kuta is very close to the airport) so you could think about leaving that for the last night in Bali instead of just a day trip. Let the forum know how it went!