There is a land train from Salou to Portadventura although if you are staying at the back of the resort it is possible to walk.
I have always been to Portadventura in the summer holidays when it was very busy, but it was less busy when the weather was rainy one day and also noticed around 5 o'clock it got quieter. Some of the big rollercoasters had no wait at all!
There are lots of shows as well as the rides. The paper leaflet shows the times. Some of them are very good. Definitely plan the day around the ones you want to see as some only have a few shows per day.
There is a parade & fireworks ending around midnight. We used to go later in the day on one day then stay until the end.
You can take drinks in but not food.
There is a water park next door which is also good.
Salou has a nice promenade with a fountain show in the evening (free) at the far end. The times were on a board, I think.
No idea about restaurants as we were half board. Food is quite expensive at Portadventura but not as ridiculous as Disneyland Paris.
There is a cheap bus to Cambrils, no need to book a trip.
There is train station in Salou & the train goes to Barcelona.
I think Ferrari land has opened since I have been. They were building it when I was last there. It is next door to Portadventura.
Have a great time!