Hi,

I have 8 hours to spend in Heidelberg before I proceed to Munich. I probably will spend maybe a few hours at the castle (or none at all if it's not recommended?). What other attractions are a MUST-SEE? I'm open to spending a night over at Heidelberg if it's really good! :)

Please help! Thanks! :)

The castle is recommended, so is a boat trip on the Neckar river. In general, the town is really wonderful to walk around in, enjoy the old architecture, pubs and restaurants. The shopping is just mouthwatering.
There's plenty to read about Heidelberg at
http://www.virtualtourist.com/travel/Europe/Germany/Land_Baden_Wuerttemberg/Heidelberg-55030/TravelGuide-Heidelberg.html

Thanks for your reply ATLC! Really appreciate it!

Another question is, is 8 hours enough for a visit to the castle, the boat ride and the walking around?

I don't know ellage. Castle and walkaround: sure.
I had a day tour boat ride, so that would take up all your time.
Heidelberg is worth a visit of longer than one day though.

Heidelberg Tourism
http://www.cvb-heidelberg.de

Generally the Castle, the old town, the old brigde.

The Neckar Valley is scenic and has many castles. This (till Heilbronn) is also the first part of the Castle Road.
http://www.burgenstrasse.de
You could e.g. travel along the Neckar Valley to Stuttgart and change their for a train to Munich. Take care that you choose a train via Eberbach to Heilbronn, not one via Sinsheim (different route).

On the other side of the Rhine, but not far from Heidelberg is Speyer
http://www.speyer.de/de/tourist/sehenswert?switch_language=en
Far less known abroad, but really worth a visit. Speyer Cathedral was the biggest church in Europe when finished nearly 1.000 years ago. And the burial place of many German emperors. Also an UNESCO world heritage.