There is so much to suggest to you.
Bob is correct, you cannot walk to Sosua. Instead walk to the highway, flag down the Gua Gua and take the bus to Sosua! Then walk around Sosua. Go to SOsua Bay, eat lunch at Rocky's.
Also another day same deal - take the Gua Gua to Sosua, change to the Gua Gua to Cabarete, spend the day on the beach.
I highly suggest the waterfalls but I do not recommend the Banana Republic Catamaran. Instead book Freestyle Catamaran. You will have a much better day! Can't comment on white water rafting.
Check out
Isaira Tours - Dominican Republic for more tour info.
Nightlife in Playa Dorada - all clubs are tourist oriented. Mangu is currently the favourite. Check out Hemingways on a Saturday night after 11.
If you want a taste of nighlife in Puerto Plata at local clubs, just PM or email me. I'll be happy to take you.