Specifically modules, scenarios and/or volumes from other AP's that can be inserted to expand the campaign without too much difficulty?
We're in the middle of Book 3 at the moment, but we've added several other modules/scenarios throughout the campaign (the players like getting more time in at low/mid levels).
Daughters of Fury between book 1 and 2 - after saving Trunau, the heroes went in search of reinforcements from Lastwall (moved the location of the town in the module closer to border with Trunau)
After book 2, inserted Down the Blighted Path - the (all-Dwarf party) heroes wanted to sell off some of their treasure and re-supply, so they stopped at the Davarn Trading Post (moved the location into Mindspin Mountains)
after Blighted Path, inserted Clash of the Kingslayers - the party sought out the Dwarven craftsmen of Glimmerhold (used to bring in new PCs to the party)
These required a bit of adjusting for party strength, but all worked well in the overall flow of Giantslayer so far.