I've done a bit of work in this arena and never encountered a product that generates Activity Diagrams.
Generated diagrams, e.g. Class and Sequence, tend to be too busy to be useful for all but trivial systems. Someone usually has to reduce them to just what is needed for some specific purpose. Otherwise it's like the proverbial sipping water from a fire hydrant.
I favor products that reverse structures (Classes and their attributes and methods) from the source code into some model and then let me draw upon that model to create useful diagrams, from a particular perspective and for a specific purpose.
I’ve had good luck with:
- Rational Rose, before IBM’s acquisition.
- Sparx Systems Enterprise Architect.
- Visual Studio and Visio (circa 2005), before Microsoft crippled the products. It's a shame they removed the round trip engineering capabilities - and more.
Personally I haven’t had much luck with free/open source products. Best of luck!