we have always had to go to guest services to get our hand stamps.
there are 2 sections, though - one outside the entrance, and one inside after you have gone through tickets etc, so check which queue is shorter 
my dds are 6 and 3, and the 3 year old is a bit of a scaredy cat, so we haven't done anythign really fast.
they enjoy:
the hot air balloon rides (there are 2 different ones)
the carousel (dd2 is obsessed with carousels!)
dd1 likes the green train, dd2 won't go on it because she had one drip of water land on her once (the rest of us were soaked!)
fairytale brook is a good one.
boat school excellent, haven't done driving school, as can't accompany dd1.
the dinosaur safari is good. we thoguht the dino rollercoaster nearby was a bit violently fast.
skyrider is ok, a bit tame.
we went on the helicopters near duplo world for the first time this weekend - they were quite fun, but not really thrilling.
dd1 has done the scarab bouncers - looke dlike fun (dd2 refused to go on it)
will have a think and see if I come up with anything else!