Going up and down or strolling around the castle you will meet a lot of rock formations. The rocks are not giant but very pitoresque in my opinion. Going up take the red hiking trail and going down take the black hiking trail. If you want not to miss the most important rocks on the hill buy a hiking map or get acqainted with the map hanging at the entrance
Go to the tower and enjoy the views. Going up- and downstairs is not exhausting. You will see a great panorama of the Giant Mountains and some Jelenia Góra districts.
For history and some details look at Chojnik Castle.
The castle is also famous for its knight tournaments taking place every year. The knights fight for "The Golden Bolt of Chojnik"- it's a crossbow tournament. If you are interested in knight tournaments you should visit Chojnik when this tournament takes place. [For any detail you can visit the sithe listed below. It's only in Polish. If you don't understand and you are looking for the dates of the next tornament you can simply write me an email and I will find it out for you.]
Visiting the ruins you can practice your crossbow and bow shooting for a couple of Zlotys.
You need about 1 to 1,5 hours to see everything.
If you are starving you can visit the castle restaurant. It serves some Polish dishes. I ordered a soup called Zurek. It was tasty and not so expansive.
About 2 Euros in 2008.
From the station in Jelenia Góra. 
Take the bus line 15 going to "Dom Hauptmanna"[Hauptmann's house]. The bus operates every hour.
Get off at the street: Bronisława Czecha in Jelenia Góra Sobieszów (Sobieszów is a district of Jelenia Góra).
You will find there a sign or simply ask. (To find an English speaking Pole is like looking for a needle in a haystack.)
If you find a sign then follow the red walking trail. You will walk a couple of minutes and you will enter the KARKONOSKI PARK NARODOWY [National Park of Giantmountains].
A Park "Ranger" ;-) will make you pay up. 4,60 PLN (approx. 2 USD - 2008)
There are a lot of possibilieties to get to the Chojnik Castle. If you are in Jelenia Góra ask at the reception desk in your hotel.