Bus ticket availability varies by company, I believe. I know I've sometimes seen tickets for buses 2 or 3 weeks in the future (possibly farther out than that), but I don't usually buy that early. Fast-train tickets for the Barcelona-Tarragona leg are on sale through May now. Renfe is quite erratic in when it places tickets on sale. You can consult Seat61.com for background information on Spanish trains.
I see that Salou is quite close to Reus--though there's a good chance you'd have to start with a bus back to Tarragona. Reus was Antoni Gaudi's birthplace and has a Gaudi Centre that's worthwhile if you're interested in his architecture. Reus is an attractive town with two modernista buildings that can be visited (for a fee). I enjoyed both the Casa Navas and the Institut Pere Mata. Note that except for the foyer, no photography is allowed at the Casa Navas (and they don't have a photo book for sale there--maybe at the Gaudi Centre). And the Institut Pere Mata is bus ride from the center of town.
I believe Morella (inland) and Peniscola (costal) both have their points of interest, but I don't know how viable they are from Salou via public transportation.
Valencia is a bit tougher than one might think. The train from Tarragona (apparently no stop at Salou) takes over 2 hours each way. The full fare for a last-minute round-trip is about 65 euros per person. By buying non-refundable/non-changeable tickets far in advance, you might pay only 21 to 32 euros per person. Valencia's a city of considerable size, so that seems as if it would be quite a cramped day-trip if you had to spend over 4 hours on the train, and that doesn't include the time you'd spend getting to Tarragona.