http://www.museumwales.ac.uk/en/whatson/?event_id=3296 though they do charge for parking, think it's £3 but don't quote me on that
There's also the train that runs along the lake ... that isn't free but is good fun http://www.lake-railway.co.uk/fares.html
The event runs from the 16th to the 19th of Feb so it's a bit of a heads up for a good day out too for anyone mad enough to either live or be camping in Wales in February
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