In reply to Oujmik:
Cadair Idris by the two routes mentioned will be very worthwhile. The Minffordd path may be quite busy in good weather, while Cyfrwy Arete is also popular and may feel harder than graded in less than perfect conditions. You may wish to have a respite from the crowds on the Minffordd path by doing a grade 3 scramble on Tapiau Llwydion above Llyn Cau. See:
http://www.summitpost.org/route/395231/Tapia-Llwydion.html and
http://www.ukscrambles.com/wales-scrambles/cadair-idris-scrambles/
If transport can be arranged, then a traverse of the whole Aran ridge from Cwm Cywarch to Llanuwchlyn (or vice versa) is a great day out.
The Rhinogydd generally provide the roughest and toughest walking in Wales, especially suitable - and rewarding - for those who enjoy long mountain days. Starting and finishing at Cilycwm to the west, an ascent of both Rhinog Fach and Rhinog Fawr on the same day will be the highlights of a surprisingly demanding and rarely undertaken anti-clockwise circuit of Cwm Nantcol. Failing that, both mountains will provide a good walk in their own right. In good weather this is a very fine place to be; a mysterious and less travelled land where an absence of crowds is virtually guaranteed. For some, like me, this is tantamount to perfection.