HMH and SMGG are the only two attractions with Halloween overlays. Other than that, it's mostly just decor in Frontierland (including Big Thunder Ranch and Día de Muertos display), NOS, and Main Street. It's not in the park, but the GCH also has the Spooky Tree confectionery showpiece.
I've thought about this a little more and my plans would depend on whether you going to DL on a morning when there is a Magic Morning/EMH or not. If there is no MM/EMH, I would head straight to SMGG and ride that at opening (the line should be fairly short), then head towards HMH and either get a FP for that or ride it depending on the length of the line. Afterwards, or while waiting for the HM FP time window, I would wander over to Frontierland and check out the decor there. If there is a MM/EMH that morning, I would grab a SMGG FP as soon as you enter and then head over to HMH and ride that and check out Frontierland while waiting for the SMGG FP.