A great stop for all the family if you're visiting, and a fantastic remedy to the boring visa trip for those of you stationed in Nagano prefecture.
The name is a little misleading, I guess. There are no fossils or anything cool like that (although the associated buildings were closed when I went). What IS there is a pretty large collection of life-sized models, scattered around the scenic hilltop park. Some of them have ladders to get inside, slides down the tail, swings from the mouth etc. It's pretty harmless fun for all ages.
There's a picnic area, and various walks through the surroundings.
Connected to the park, near the upper parking area, is Nagano Chausuyama Zoo. I didn't go in, but it reportedly has 57 species of animals and charges a mere 500 yen entry (and only 100 for kids). It also looks like you might see a porcupine wandering freely around the grounds.
The dinosaur park is open all year round (as there is no gate, but there looked like there was a closed gift shop and potentially a small museum when I went), but the zoo is closed mid-January until the end of February.
It's a little confusing to find thanks to sporadic English signs, so here's a map.
View Chausuyama Dinosaur Park and Zoo in a larger map
There are a lot of other things to do in Nagano city and the surrounding area, so be sure to check out our Nagano Prefecture clickable map for the full range of options.