Tarragona is amazing. We came back from Salou this week and it was a higlight of the holiday.
Get a t10 card from the Plana bus shops in Salou (easy to find) and it's about 15 Euros. Everyone in your party can share the card and each credit is one trip for one person. Really makes it cheaper than paying for fare on the bus. The Roman amphitheatre is ace (you pay a small charge to go down to it) and the Cathedral is well worth trying to find. It was 5E for an adult and 3E for children to enter and was a beautiful place to visit.
The park is fun. Def aim to stay for the end of the day parade (starts at about half 11) and fireworks. It's worth finding somewhere good to sit and watch them by 11 at the latest as it gets busy. The action all happens on the lake itself so it's easy to find space.
The shows in the park are good. My DDs loved the fire throwing which is about 9 at night. Again, aim to get there a bit earlier as it fills up fast - we never got a seat for that one!
Water park was good. If you don't have your ticket yet, wait to buy it as you leave the park one day as it's cheaper by 30% to buy on exit from PA park rather than online. I had a bit of fuss because my 4 day ticket hadn't yet expired so they wouldn't sell to me, but I queued up at guest services at the entry to PA and they sorted it for me no problem. We didn't get a sunbed but don't do sunbathing anyway, so no loss. Instead we were able to find shade under canopies on fake grass and laid down towels to lounge on instead. Food not too much of a ripoff. They do check bags, but they let you take in water bottles (and didn't think of this myself) if you wrap a small amount of food/snacks up well in your towel they prob won't notice.
The fountains in Salou on the front (a lovely walk with the palm trees) are cute, but kitch. Popular though!
Burger king is on the front. Really easy to find. And right next door is macdonalds. Which is handy if you want to be able to get 2 for 1 entry into the Aquopolis water park in La Pineda (a bus ride away) as long as you've downloaded the app.
Michelangelo's is a well stocked supermarket that doesn't exactly say supermarket on - unlike the one we aimed for when we first got there that only really sold crisps and drinks!
We loved it. So much that we've already booked for next year as it ticked so many of the boxes for me (hotel/beach/historical town) and the kids (pool/park/waterpark).