The main reason to go to Pompeii is of course the ruins which are out of this world. It is amazing how much that has been recovered. The minimum guided tour is about 3 hours and the maximum is 8 hours and that one doesn't even cover the whole site so if you are going to go round on your own, I would give the ruins at least 2 whole days of your trip. The site is huge so wear
You can't go to Pompeii without a trip to Mount Vesuvius which you can either set up online before you travel or your hotel can organise it for you, we did ours through our hotel as it was easier. We were picked up from our hotel and driven up to the Vesuvius ticket office where we brought our tickets. We got back in the car and was taken further up Vesuvius to the car park where we were dropped off and told the time that we needed to be back for. They tell you it's not far to the top from there but it's not far if you are a bird, it's at least a mile walking it as it's a pathway that goes around Vesuvius. If you aren't very fit, get a walking stick, it's only a euro and it does help. There is a little shop at the top to buy drinks and souvenirs, then you get to enjoy the view from the top.
The Shrine of the Virgin of the Rosary of Pompei is a beautiful huge church in the town square. You can walk around the building that is attached as well and it's free to get in so I would say to set aside about half a day to see it all. Maybe a bit more if you want to stay for a service.
Hotel Forum Pompei is a fantastic hotel, perfectly located across the road from one the ruin entrances. It isn't very expensive to stay there and it feels like real luxury. Breakfast is included which is buffet style with different cereal, breads, hams, cheeses, etc and on nice days, you can sit in the garden to chill out while you eat.
We love Pura is the most amazing place I have ever had gelato, not only do they the best gelato ever but you can then sit on a swing to eat it! I had a great hour sitting at the front of the shop enjoying hot chocolate and a bowl of gelato while sitting on a swing and people watching.
The Old West is a fantastic restaurant that everyone should try. They have a great menu with yummy food and the portions are huge! Plus, they have an awesome cocktail menu and loads of drinks memorabilia that covers the walls.