If you plan to go on the Snowdon Mountain Railway then get to Llanberis early 'cos it books up quickly. You'll also need
lots of dosh - £20 for adults, £16 for kids.
http://www.snowdonrailway.co.uk/
The slate museum in Llanberis is good and, if I remember correctly, free!
http://www.nmgw.ac.uk/wsm/