http://robotsquare.com/2012/03/03/tutorial-official-bonus-models/#11
It's a good way to get ideas, and start off if you just bought the set.
#1M.O.R.P.H
I'll have to say this is one of the more complicated builds and programs.
#2Llama
This one is easier than others, and also very entertaining for younger children.
#3Manty
Also quite simple, but very fun and children may also like this one
#4StoneHenge
More complicated build and program.
#5Segway with rider
Easier build, programs pretty simple too.
#6NXTitzki
Children like this one, a little harder than Manty and Llama, but easy programming.
#7ArchFish
Another kid-friendly build, simple and fun
#8TriPod
More serious build, but interesting and pretty simple
#9MindCuber
Great build for those who want thier rubix cubes solved. Complicated build and program
#10Pinball
Simple and fun build.
#11Color programmable car
Easy build, more complicated programming.
Videos:
Here are some videos done on the NXT
Not all of these videos where done by the maker of the Models
1# M.O.R.P.H
2# Llama
3# Manty
4# Stonehenge
5# Segway with Rider
6# NXTitzki
8# Tripod
9# MindCuber
10# Pinball
2# Llama
3# Manty
4# Stonehenge
5# Segway with Rider
6# NXTitzki
8# Tripod
9# MindCuber
10# Pinball