Hi Freewillow,
I've added land-based diving before/after four Galapagos liveaboard trips, doing dives from Santa Cruz, San Cristobal and Isabela.
One of the best dives was Kicker Rock out of San Cristobal but my understanding is the other dives out of San Cristobal are not of the same quality.
You'll find greater diversity of dive sites diving out of Santa Cruz, including Gordon's Rocks, the best land-based hammerhead site.
If you have not already done so, I would also recommend allowing a couple extra days for land tours beyond what the liveaboards visit.