"The Secor book has gotten me way too excited. :)"
With good reason. It's a candy shop out there for such as we.
Darwin Canyon would be an easy 1 day side trip off the JMT. You wouldn't have to tackle Lamarck Col to do it if you're coming south on the JMT.
Davis Lake/Martha Lake from Wanda Lake. I've done this route, exiting via Goddard Canyon. It's a neat route, particularly the descent of N Goddard Creek. The only problem is that exiting via Goddard Canyon would force you to retrace you steps once you got back on the JMT. This would be a hard 2, reasonable 3 day side trip unless you're a real hard core type. If you decide to do it, be sure to stay on the north side of Davis Lake to avoid a cliff band on the south side.
If you go visit 60 Lakes Basin, it would be worth your while to poke around in Gardiner Basin while you're at it. It's really pretty back in there. I understand there is a way trail that goes from Gardiner pass over to Charlotte Lake where you could pick up the JMT again. When I was in there, I bushwhacked up Charlotte Creek from Bubbs Creek and exited via 60 Lakes, so I can't vouch in person for the Charlotte Lake/JMT connector trail from Gardiner Pass, but it was mentioned in a trip report here a few months ago. You might want to look that one up if this sounds interesting. It would be a 2 day side trip, 3 if you fell in love with the place.
The Kern Loop. I'm not familiar with this name, but if it describes the loop from the JMT to the Upper Kern Basin via Lake South America and back to the JMT via the connector trail from the Kern Canyon Trail back to the JMT, I'd very highly recommend this one. You could do it in 2 days, but adding a 3rd day to explore Milestone Basin would be well worth your while. It's a gorgeous area, my favorite in all of the Sierra.
"Either the PCT or Crabtree Lakes->Crabtree Pass->Miter Basin"
I'd advise Crabtree Pass. It's a much nice route. A bit of scrambling up to the pass and some beautiful lakes on both sides of it. Lots of bears in the Sky Blue Lake/Miter Basin area. Be on your toes.
Edited for content: One thing just occurred to me. If you end up doing the Wanda/Davis/Martha loop, you'd be wise to inquire about stream conditions first. The crossing of either N. Goddard Creek or the S. Fork of the San Joaquin could be problematic if they are running high, especially the S. Fork. If they are running high and you still decide to go, consider crossing North Goddard Creek higher up. This will allow you to follow the S. Fork of the San Joaquin up to Martha Lake(~ 1 mile) and cross it at Martha Lake. From there on down Goddard Canyon it is trail all the way to the JMT where there is a bridge. For my money, I think the Kern Loop is the better choice on aesthetic qualities alone. My 2 cents.