Select Motor Racing, Grandstand Motor Sports, Motor Racing International all do packages, however, depending how confident you are as a traveller, Monza is easy to do on your own.
We have stayed in both Milan and also on Lake Como (Bellagio and Menaggio) and the transport to and from the circuit is excellent. From Milan there are free trains on Friday, Saturday and Sunday from Milano Centrale station to Monza station. They get very busy, but all part of the experience. From the 2 resorts in Lake Como, you take the ferry across to Varenna then catch the train to Monza. At Monza station there are free buses to take you to the circuit. The only thing you need to bear in mind is that the buses stop in a nominated parking/drop off point which is about a mile walk into the circuit itself (Monza park is massive.) Not too much of a problem going into the circuit, but coming back the queues are quite long, but very organised.
Only downside to Monza is the toilets – we met a couple a few years ago who said they had been to festivals, so no problem – even they admitted they were probably the worst they had experienced ! Also if it rains, beware, as it is in a park it turns into a swamp.
Love Monza as we can combine a holiday in a beautiful part of Italy with an F1 fix!