Edinburgh to Conway and back in 5 days

On the way to the Lakes District, you can pick up a car in Penrith (Rick's book mentions that Enterprise has an office there) or would find more car rental choices in Carlisle (1.25 hours by direct train, departs about hourly). Returning from York to Edinburgh is easy (2.5 hours by direct train, runs twice per hour). If these are your only two train rides, then the cheapest way to buy tickets is 2 months in advance, or lesser discounts can be available at few days ahead, if not sold out; see tips and link at http://www.ricksteves.com/travel-tips/transportation/trains/britain-rail-passes#ticket-tips.

It's Conwy. The Spanish and Italians had first dibs on vowels and the Welsh wound up with the slim pickings at the bottom of the barrel.

You don't want to pay for both a train and a car on the same day, so start off by driving (train comment later). Enterprise tends to be higher - - use kayak.com - - Turnhouse and Waverely tend to have about the same prices, an automatic is more likely at the airport, but the chances are pretty good at the train station. Chances diminish in smaller towns and the prices climb steeply for anything. You can see the Enterprise sign at Penrith from the freeway which is about a mile from the train station - - I've no idea how you'd make the connection..

It's almost a wash in time between driving and the train to Penrith, but I wouldn't enter the Lake District from there, anyway. Shoot for entering the park at about Aspatria, which is on the north end. You've got two choices to get there: via the A74/M74 which is boring and can get choked up or via the A7 which has good scenery. Depending on the freeway traffic, that could be a time wash as well. You should be able to chug into Keswick in a tad over three hours of driving time from Edinburgh.

Now it's time for decisions since you don't have a lot of days for the project.

The villages, boat rides, and walks of the Lake District could use up all your days. In your case I'd give it that first afternoon, a night, and the next morning. Spend the night in Keswick and think about getting out to the Castlerigg circle - - it's a beauty. You could walk, but going out it's a mean uphill climb - - just drive. Leave heading out the next morning toward Kendal so you'll run the length of the park, but plan on a stop at Windermere.

Not counting stops, you should be able to pull into Conwy in about three hours. I'd stop for the castle (two hours at most ?) but not spend the night. The town doesn't offer much compared to what's coming up, and the castle I'd rank as about number four in North Wales. Press on to Caernarvon (thirty minutes). The town as a lot to offer for a couple of nights and has what I'd rate as the second-best castle in the area. (Castle number three is Beaumaris out on Anglesey, but you don't have time for it.) Spend two nights. For the one full day, head out on the A4086 (?) (may be the road number, but it passes just to the west of a couple of lakes in the Snowdonia National Park. You'' be wanting to make a couple of stops. After about fifteen miles from Caernarfon (part of which will be after you switch from the A4086(?) to the B4410, you'll hit the A487 - - hook a left and then a quick right onto the A496. After about five miles you'll come to a fork in the road. The sign says Harlech to the right and crosses a stone bridge - - take the left fork and stay on the wider road. You'll come into Harlech the back way. That's the best/strongest castle in the Principality and number two in the world. It's only going to take about an hour. There's several places to eat in town. When you leave the castle drive down the hill toward Irish Sea and admire the castle up on the cliff. Leave the area on the A496 - - you'll eventually come back to the stone bridge you didn't cross before and backtrack about five miles to the A487 (which you've also seen before, but this time you'll stick with it heading west down the other side of the Afon Dwyryd toward Porthmadog and follow your nose back to Caernarvon. Total driving time for the day is less than three hours. Sorry about all the road numbers, but there's just no villages out there to point to. The route will be readily apparent once you glance at a map.

York is about three hours from Caernarfon. Haul tail, there's not much to see along the way. You've got two nights left over. Ditch the car on arrival and take the train back to Edinburgh when ready.

That concludes what you asked for. Now a personal opinion:

You wanted a train ride; you got it. What I'd do is drive back to Edinburgh because there's so much to see along the way. It's four hours up the freeway. But, if you cut over toward Scarborough you can hit Whitby, the North York Moors National Park. Blythe, Alnwick Castle, and a dozen other places that you probably haven't thought of. Hugging the North Sea coast as much as possible, it might be six hours of driving, but it could be shortened depending on how much time you want to devote to the project. It's easily worth an overnight.

Full Disclosure:
. I don't know how much time I've spent in Scotland and Wales in my life, but I wormed in two months in Scotland last year and finally finished walking the perimeter of the Principality.
. I don't care for the Lake District except for walking - - my wife really enjoys the villages.
. I think York is worth a couple of hours - - my wife thinks it's worth at least a day and a half.

Following on from what Ed says, I would visit Portmeirion when doing his Caernarvon to Harlech trip & near Portmadog. See > http://www.portmeirion-village.com
Also check out the steam railway from Caernarvon to Portmadog = www.festrail.co.uk
Near Conwy check out Bodnant Gardens = http://www.nationaltrust.org.uk/bodnant-garden/

If driving from York back to Edinburgh, consider visiting Bamburgh Castle on the North Sea coast.
www.bamburghcastle.com
Also in Northumberland is Alnwick Castle = http://www.alnwickcastle.com

Being as you are already in Scotland, have you considered just touring highland Scotland? If doing Scotland, my choice would be to drive out to the Isle of Skye & devise separate outward & return routes to Edinburgh.
www.visitscotland.co.uk