Kaggle is a pretty great place to evaluate your R or Python skills with tons of datasets for you to explore.
You can also sign up to data science competitions.
In terms of data wrangling in R, I used the Global Terrorism Database dataset that can be found on Kaggle.
You will have to sign up to Kaggle in order to use the dataset.
I have written the code here with tons and tons of comments.
The code is written such that it is easy for people to follow.
Let me know (via email) if you prefer to have the dataset on your local computer instead when working through the code.