In reply to Sharp:
Difficult to advise without knowing your nav experience and walking speed. Have a look at the Carneddau, the Glyderau, the Nantlle ridge and the Rhinogydd.
Planning a circular route rather than simple 'out and back' can be the tricky bit.
The only places likely to be truly 'crowded' are the main paths and summit of Snowdon, and maybe North Ridge of Tryfan, particularly at weekends.
P.s. back of car sleeping's not a major issue in North Wales if you can park away from traffic.
Post edited at 16:16