I should write some kind of application in a bunch of different programming languages to get a proper taste for them. Hmm, maybe an IRC bot.

Writing an IRC bot means you get to see whether the language can easily be used for "real-world" use-cases (TCP/IP networking and filesystem access being typical litmus tests IMO), it lets you see how well you can mangle strings and bytes and work with Unicode in the language, and you can work with a couple data structures like to make the bot remember things. You can make it extensible with additional feature modules to test how well things compose in the language.