We've been there a few times and stay at the Hershey Lodge. It is a little overpriced, and the service isn't stellar, but there are a lot of dumpy motels in the area, and this is one of the nicer ones.
Chocolate World is a must do. It is adjacent to the park. There is a 3-D show that is new and sort of compares to something like Muppet Vision at Disney. You have to buy tickets to see it.
There is also a free ride in Chocolate World that takes you in a car through a talking show (sort of like the Great Movie Ride or Spaceship Earth). There is no charge and you learn about how chocolate is made. You also get free chocolate bars at the end of the ride!
Inside Hershey, there are lots of rides and attractions, but I wouldn't say anything really stands out as being a must do at Hershey--not to me anyway. It is a nice, clean theme park like Disney (NOT like Six Flags) and has a mix of newer rides and older, more carnival-like rides. Overall, it is a one-day tour and there is something for everyone.
It is a little known fact too that there is a mini-zoo at the back of the park with a separate entrance and the price of admission is included in your regular Hershey Park admission ticket. I think most people miss this, and it isn't really very crowded. It is called ZooAmerica and is themed to different areas of the United States, so there are no exotic animals here, but it is still a good chance to get close up with some bears! The zoo isn't very crowded either.
Also at Hershey--next to the park across from Chocolate World is the Hershey Museum--educational and fun.
Also, at this time of year, you would want to check out Hershey Gardens. You can grab a free bus there from either the Hershey Lodge or the park and stroll through gorgeous landscaping and collections of rare plants and flowers. There is also a butterfly exhibit in the summer. We haven't seen that, but it is supposed to be very beautiful.
At the Hershey Lodge, there is an indoor swimming pool, outdoor pool, miniature golf, a few restaurants (pricey), gift shop, and organized games for kids (although they aren't all that organized).