I have spent several weekends in Wales, chiefly in Snowdonia. I have climbed Snowdon (or Yr Wyddfa) by 6 different routes and it remains one of my favourite mountains - not least because it has a restaurant on top!
The only route which is less than interesting is the most popular - the railway track from Llanberis. Its a quick and easy way down in the mist but otherwise has little to commend it.
My favourite route is to go over Llydaw to Snowdon Summit, enjoying the exposure and picturesque views of the lakes below, then down by Crib Goch. The final knife edge section of ridge really is the icing on the cake and in a strong wind reduces most sane humans to all fours!
Llydaw from the Pyg track
Crib Goch towards Snowdon summit
If you prefer your walking to be of a more relaxing variety then try the Watkin path. A beautiful trek up a delightfully green Welsh valley culminating in a slog up the scree to the summit (part of the Llydaw route too)
Just across the valley are the Glyders - although my only attempt on these hills (Fawr and Fach) ended in a white-out when I was attempting a solo ascent. After 2 hours of navigating by taking backbearings on my rucksac and the visibility down to less than 20 yards I gave up. (Partly based on the fact that I knew the summit wasn't far from the edge of a cliff!)
Behind the Glyders lies the great plateau of the Carneddau. I raced round this in pouring rain one September. Apart from getting soaked I can't remember too much about them!
The only other hillwalking I've done in Wales was on a management training Outward Bound course at Aber Dovey. We had a midnight hike over the local hills looking for clues in a 24 hour who-dun-it game challenge. The hills were only about 1000 feet high and we weren't looking to reach the summits but it was an interesting few hours in sub zero temperatures trying to locate items by map reference then decipher clues based on these - great fun!