In north Wales, if you started at Llandudno Junction or Conway you could walk the length of the Carneddau. The peaks are high and the main ridge is easy walking in decent weather. Plenty of wild camping opportunities there.
Basic provisions are available at Capel Curig.
You could continue via the Moelwyns. The peaks are not the biggest, but there is lots of wild country to get lost in.
If you prefer the big peaks you could take in the Glyders and Snowdon instead. They are very popular and not ideal for wild camping, but discreet camping higher up should be possible.
The Rhinogs as mentioned are also good for wild camping. The terrain is rather fierce; the rule of thumb for the Rhinogs is find a path and follow if.
For a finale you could climb Cader Idris.
You won't have time to do all this in 5 days. However there are the Ffestiniog railway and the Welsh Highland railway to expedite your journey. As mentioned there are buses too.
You could return by train from Porthmadog or Barmouth.
It depends what most appeals to you. Are you looking to climb mountains or are you more interested in a semi-wilderness experience? In general, north Wales has the better mountains, but mid Wales more wilderness.