Schadenfreude is the joy or pleasure derived from the misfortunes of others. What is the word for joy or pleasure derived from the happiness of others?
Seeing just the title of your question ("What's the antonym for Schadenfreude?"), my answer would have been "Mitgefühl" (to keep it in German) or "compassion" (English), since I'd say that Schadenfreude is the absence of compassion.
Having now seen your description as wanting a word to express "joy or pleasure derived from the happiness of others", I'd say "Mitfreude" would be it in German, and "sympathetic joy" would be the closest I can think of in English (couldn't find a single word, though a bit of googling did turn up "Mudita" as per cornbread ninja's response, so +1 from me).
Mitgefühl means "sadness derived from the sadness of others".
Schadenfreude means "joy derived from the misfortune of others".
Mudita/Mitfreude means "joy derived from the joy of others".
Mudita is the Buddhist concept of joy.
It is especially sympathetic or vicarious joy, the pleasure that comes from delighting in other people's well-being rather than begrudging it.
I'm not familiar with mudita, but this 1871 definition says it's the meditation of joy, but ... not the joy arising from earthly possessions. Personally I'm not sure it's really an English "word", nor does it seem to be (in its original sense, at least) particularly associated with vicariously experiencing pleasure directly experienced by another individual. In its Buddhist context, the priest aspiring to it should already have transcended concepts of individuality anyway.
You could use the word Compersion perhaps.
The feeling of joy one has experiencing another's joy, such as in witnessing a toddler's joy and feeling joy in response.
As the Corndog Ninja noted, mudita is the concept of finding joy in the happiness of others.
If you want a rough German antonym of Schadenfreude (or simply schadenfreude in English texts -- "enjoyment obtained from the mishaps of others," as Merriam-Webster defines it), then Seligkeitfreude would work.
The word confelicity has been suggested for this concept, most recently by Susie Dent - however, real-world usage of it (outside of collections of obscure words) seems to be scant to non-existent, and it isn't found in any mainstream dictionaries.
From the Latin prefix con- (from Latin cum (“with”)) + the Latin felicitas (“happiness”).
(rare) Pleasure in another's happiness.
Unity or agreement of feeling or action, especially among individuals with a common interest; mutual support within a group: ‘factory workers voiced solidarity with the striking students’
In Spanish this word has a strong connotation with empathy. I'm not really sure if it would be understand the same way in English.
Also, you can say,
Lisa always celebrates others' successes.