Not crazy as a dayhike at all. Muir is probably the easiest add-on that I can think of. It is literally several hundred yards away from the trail on the West side. This is how close it really looks from the trail:

http://www.summitpost.org/the-way-up-mount-muir-from/13800
http://www.summitpost.org/hiker-walking-by-on-the/24312/c-150495

This is probably the best photo I've seen showing you what the route looks like. If you're comfortable with that sort of scrambling, you'll be fine. If it looks scary, it's best to stay away.

http://www.summitpost.org/mt-muir-3rd-class-route/364878/c-150495