Salou has a fantastic bus service - use the buses to take trips to places like La Pineade (for the water park), or the Bauna Vista market rather than using the Rep's organised trips. it'll save you a fortune. ( Airtours were charging something like 8 euros each for the market, but it was only 3.80 euros return on the bus when we went last month!)

Hi,
A Taxi will cost you about £15-£20 each way, or you can get a bus from the airport into salou but i dont know how much it is.
The waterpark at La Pineda costs about £20 adults and £11 for kids, but the price differs as it depends what you want to do, you are charged extra to see the Dolphin and sealion shows. There is a market on behind this waterpark every friday. Also in La Pineda was crazy golf 1.80 euro for as long as you want, which was the cheapest we seen, the kids were amused for hours (if you want to know where just ask)
Check out http://www.portaventura.com for the parks prices, it may work out cheaper to go to the Costa Caribe waterpark when you buy your fair tickets.
Boat trips you can book without going through the reps, it will work out cheaper. Go to the Marina in Salou or to Cambrils Lots of boat trips leave from there. If you find a information kiosk in Salou you will be able to find out better.
If you like beaches then Salou beach is perfect for spending a few days of your hol, its shallow water makes it really safe for kids, buy them some fishing nets and let them catch the fish that come into the shallow water to swim around your feet I was scared but the kids loved it
I am sure that you will find plenty to entertain you.
Have a good holiday
Loulou.

OOps forgot to add
Try http://www.salou.co.uk you may find more info on stuff to do
and if you use the public transport watch out because after 6pm the cost of the busfare doubles so it may be cheaper to get taxis for you.

Loulou