Oh well where do I start. Take the toll road to Harlech, the views are beautiful, Harlech has the most fantastic beach, further on down the coast you will come to Llanbedr, take a left to Cwm Bychan into the mountains spectacular views, this is where 'inn of the 6th Happiness' ws made with Ingrid Bergman - drive through Llanbedr to the other side of the village and take a right turn down to Shell Island, wonderful place, spot the seals. Continue along the coastal road and you will eventually reach Barmouth and lovely seaside tow (buckets and spades, and a few penny slots thrown in).
Im only mentioning a few, you've already talked about the railway in Porthmadog, head towards Black Rock sands and theres a place where you can go Quad biking.
If you want night life you will have to head towards Caenarfon and Bangor with its University's and students, cross the Menai to Anglesey and take a gander at the lighthouses, or the beautiful craggy fishing villages.
Theres so much to do, and if you have good weather well there's nowhere else more beautiful in the world. Disney is excluded here, cant be compared.
ENJOY North Wales, I live here.