Good morning to you Roosso.
Not knowing your grandson's age puts me in a dilemma as to what he may or may not like, however he goes. May I be so bold as to suggest travelling Eastwards would be beneficial in terms of breaking up a long journey across the pond. Also it is relatively easy once you have your hire car to join the motorway network at Manchester Airport and head West to North Wales.
First stop really should be Llanberis for at least one night, there is plenty of Welsh Slate mining heritage on view with especially the Slate Museum well worth a visit, Michael will testify to that as he paid a visit this Autumn.
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museum.wales]
There are also two narrow gauge railways based in the town, namely Llanberis Lake and Snowdon Mountain Railway
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snowdonrailway.co.uk]
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www.lake-railway.co.uk]
It's about an hours drive to Porthmadog where you really should spend a couple of days at least. Obviously the attraction is the F&WHR with it's two railways and as you have seen from the pictures in this link, the engines, coaches and scenery are wonderful. I believe Easter is early April next year, so avoid that at all costs, Michael will add his knowledge and opinion on driving around the area, Denver or LA it most certainly is not.
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www.festrail.co.uk]
So I guess that narrows it down to end of April early May before the holiday season really gets going. If you have the family with you perhaps the following link may be of assistance, I use these guys for house rental close to the F&WHR station. Michael can also vouch for them as he used STS on his trip this year. The apartments on the Wharf are really very close to the railway.
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www.sts-holidays.co.uk]
If you decide to take the railway to Blaenau Ffestiniog or take a drive then close by is the Llechedd Mine, your grandson well may enjoy this one.
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llechwedd.co.uk]
One last attraction worth a visit, and not with the grandson particularly in mind is Portmeirion Italian village at Minffordd close to Porthmadog.
The drive back to Manchester Airport from Porthmadog should take you in the region of 2 hours.
Hope this is of assistance, know doubt Matthews report and Michaels pictures will showcase the F&WHR , oh I almost forgot the Talyllyn Railway at Tywyn which is about an hours drive South from Porthmadog
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www.talyllyn.co.uk]
And finally there are plenty of flights from Manchester to Frankfurt and Zurich each day. Any further questions and I'll do my best to help.
Bob