When I have gotten past the basics of a particular language, I start to think about other play projects I've worked on and try to decide how to reimplement them in the new language. Things I like to try are
- building a binary heap
- building a simple HTTP library
- implementing various sort algorithms
- parsing an RSS file
These kinds of little projects for me help me to get a gauge for what the language is like and what its strengths and weaknesses are. It also helps me to understand the gaps I currenly have in my understanding of that language.