I think its been mentioned in this thread, but i recon you would enjoy the 3 peaks challenge. Snowdon, scafell and ben nevis in under 24 hours.
I did it a few years back for charity, really enjoyed it, and its quite well organised.
Once you get ben nevis done, which is a cracking walk, you might want too look at the 3 peaks.
http://www.thethreepeakschallenge.co.uk/