Sion

If you are just staying in the town of Sion there are a few "must-sees". Hike to the top of the two hills in the middle of the town. One is topped with a castle and the other with an ancient church. Both are well worth seeing. The church is virtually untouched since it's construction in the 1100s, except for some later additions in the 1400s. The oldest pipe organ in Europe is housed there and it is still played on a regular basis. The church has quite a bit of original paint inside and its share of ancient graffiti. It's well worth seeing and photographing.
The Chateau de Tourbillon on the adjacent hill is partly in ruins, partly intact and very picturesque. It dates from the late 1200s - early 1300s. It also has an small chapel dating from the 1400s with many of the original frescoes intact. If you're lucky enough to get there when the caretaker is there, he will let you in to a small antechamber in the chapel to view the pieces of the frescoes and earlier art that has been preserved.
The town itself is beautiful with a lovely old quarter to wander. The surrounding vineyards and mountains make the setting a photographers dream! The vineyards on the hills around the entire canton of Valais look like green quilt squares with the alps towering above.
I think that one night and two decent days on either side should be enough.
There are many other chateaus and fortified towers up and down the valley that are hardly visited and very photogenic! For a magnificent view of the surrounding mountains, drive up or take the Post Bus up to Haute Nendaz/Gentianes and then the gondola to the top of Mont Fort. One of the best views in the alps on a clear day! Have a drink in the tiny hut that is the highest bar in Europe.