Lyell is a great climb- a LONG dayhike, or not bad if a camp is set up somewhere on the way. Maybe 100' or so of 4th class from the Lyell-Maclure saddle. An amazing peak at any rate with phenomenal views.
The N face of Mt. Ritter from the Ritter-Banner col is quite stimulating, & the area is one of the most amazing in the Sierra. You could tag Banner the same day for a double. Camping at 1000 Island Lake is phenomenal.
Another suggestion, not as difficult as the suggestions recommended above, would be Parsons Peak. Hike up Lyell Canyon (same initial approach as Mt. Lyell), hang a R halfway up. Camping in Lyell Canyon would be sublime (check out permit restrictions, etc.). Views from the top of Parsons are breathtaking- some of THE best views of the Yosemite backcountry. Also, you'll spend considerably less driving than some of the other suggestions. http://www.summitpost.org/mountain/rock ... tpost.html
As also mentioned, ascent of Mt. Agassiz isn't too bad, & the views from the summit are spectacular. You could do this as a dayhike or set up camp en route, Bishop Pass or before.
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