Hey Kristin, congratulations!
I know a lot of Rotation churches that would be jealous of your 90 minutes. We hear the opposite: "only have 30 minutes" and such. (Often because their worship time is rather concurrent with Sunday School --letting the kids out at the children's sermon to go to class.) Makes it hard to do cool stuff.
If I had 90 minutes, I would do a 15 minute "gathering time" before workshops. Songs, worship, skits, announcements.
Most of the WT lessons are designed for 40-45 minute classes, though some workshops need more time and will often say so. Browse through a bunch to get the idea.
MOST of the Writing Team lesson plans also now have "time adaptations" ...especially the newer new ones. Many WT workshop lesson plans can easily fill 50-55 minutes if you take a leisurely pace through things (another luxury most Sunday morning programs don't have).
Let us know how it goes and feel free to ask for help.
Neil
