Food and drink is very expensive in Rome, but you can find some very nice cheaper cafes and restaurants if you come away from the main tourist areas.
There are hundreds of shops selling souvenirs but mostly selling the same kind of things, rosary beads and religious memorabilia.
You will never get to see everything in Rome in just a few days, but the easiest way to get an idea of the city is on an open top tour bus. They charge about 15 Euros and the ticket last for 24 hours. They will give you headphones which you plug into the socket beside your seat, and then you can listen to a running commentary of the places of interest that you are passing. Just jump off when you get to a place you want to visit and back on another bus whenever you feel like it.
St Peter’s is amazing. Try to avoid the weekend as it is extremely busy, but during the week hardly any queue. You have to go through the same security as you do at the airport to get in. If you go up to the dome, remember there are hundreds of steep steps to climb. They have a lift but this only takes you to the base of the dome. Do not forget to visit the shop run by the nuns inside. Where you will be able to buy rosaries that have been blessed by the pope. And look out for the Swiss guards as you leave the Vatican. They look really colourful and smart in their Orange, White & Blue Uniform.
The Colisseum will take your breath away. It costs 12 euros to get in but the queue is horrendous. For 20 euros join a guided tour. There are lots of people offering them outside. But make sure you choose someone who is wearing a licensed badge. You will not have to queue to get in, and that in its self is well worth the extra 8 euros. You will get a full guided tour of the Colisseum, The Palatine and The Forum which lasts about 2 hours. And if you’re lucky they will give you a ticket for a free tour in another part of the city.