Just to muddy the waters a little more... as you have 3 weeks you could look at slightly more remote locations.
What about Raja Ampat at a resort like Papua Explorer, or similar... your hubby can kayak, snorkel, go hiking in the jungle, go bird watching to see the birds of paradise, go visit local villages, relax in a hammock on your private over water balcony watching the sunset and reading a book... just a thought.
On the "safer" side of things, as discussed above, Bali is a great place to go on holiday. The diving is great and there are so many places to visit and things to do in between. I would look at spending 3-5 days in the followings places: Candidasa/ Padang Bai area; Tulamben; Permutuan area to also dive Menjangan Island; Nusa Lembongan; and then Ubud to chillax.
I agree with the above, N Sulawesi is great for diving (I've been 6 times), but I doubt it would be very good for a non-diver. Unless he is really happy to chill and do very little. Lembeh has very little to do and snorkeling is poor, Bunaken would be much better to great for snorkeling but again not much else to do topside, and same same for Bangka. Luko did mention a few topside activities that would definitely be worthwhile if you are dead keen to hit the area.
If you are super adventurous and you are there are the right time of year, you could look at making the trip to the Banda Islands. Spend some time in Ambon and then head across to the islands for diving and history.... just a thought.