There are 315 steps leading up to the castle - do I need to say anything else??
Equipment: Good shoes!!!
Although there is an alternative route, I would always prefer to take the stairs: it is much faster and it makes you feel so good, when you see the 300 - mark painted on the steps!
For hockey fans, be sure to visit the new (opened in 2005) SAP Arena just East of Mannheim to see the Adler Mannheim take on opponents from across Germany and Europe. Rowdy crowds, lots of singing, good food and a normally excellent team make for a great night out if you're a sports fan. The arena has it's own train stop on the Mannheim-Heidelberg leg of the local trains, and tickets are cheap. Well worth the side trip, and a great start to an evening out.
Equipment: Ear plugs, if you don't like noisy fans.
It's not sooo special, but very nice for basketball fans to go in Neunheimerfeld, on the University fields; a lot of good people come there, especially during the evenings.
Equipment: You don't need a ball, everyone has one there.
As with most German cities, Heidelberg has a lot of biking paths, both within the city limits and connecting the neighboring villages...It's a nice way to spend a sunny afternoon.
For a nice challenge, ride your bike up the hill to the Castle...you will be sucking wind by the end.....
Down by the rowing club on the opposite banks of the Neckar, you can find pedalo boats to rent yourself April to September if you want to try out the currents.