Walk around any of the back streets away from the rest of the tourists and you should find some nice still canals with some buildings/bridges reflected.
As for sunset, I have had a few good ones with San Giorgio Maggiore as a backdrop. I've mostly gone around the end of October though, and to get the Sun in a good position you may have to go quite far towards Arsenale or even to Giardini Biennale. From that area, because of the position of the Sun, you may be able to get the Sun behind the Santa Maria della Salute too.
I use an Android app called Exsate Golden Hour which can show you where the Sun will be at different times of the day. Very handy if you are on Android. I'm sure there are similar apps on iOS. It is only a guide though, as for one trip it had the Sun dropping behind the Ponte Vecchio in Florence straight inline with the river, unfortunately the Sun went behind some mountains in the distance first.
You can only try to plan your position in advance so much.
Fingers crossed you have a nice sunset.
There are some sunset pics with San Giorgio Maggiore within my gallery of Venice pics
here, to give you an idea. But as I said I went about a month later than this, or in April. There are also other pics of the quieter areas of Venice, and they all have the location where they were taken if some places catch your eye.
Please let us know how it went.
Oh, and I forgot to add, if you are there after sunset, there are some nice pics to be had from a jetty that juts out into the Grand Canal to get pics of the Rialto Bridge. Tripod recommended for that one though.