Maybe some sort of DOS utility for older PC stuff?
You can try writing simple games: Blackjack, "Slot machine", Battleship. Simple games. Use some character graphics (pretty sure the PC character set has the 4 suit characters). My teacher wrote a very nice blackjack game on the PET long ago.
You can try writing a simple DOS utility. Try to write a crude clone of Midnight Commander. This gets you in to simple DOS calls (scanning the directory, removing files, renaming files), sub directories, things like that. Feel free to mercilessly copy UI examples.
Write a SORT program. First you can write one that just loads a text file in to memory, sorts it, and writes it back out. Then you can modify it to define simple fields (i.e. character sort on characters 1-6, or 10-15), add a flag to ignore case. Then change it to work on files that won't fit in memory. This gives you a quick dive in to sorting algorithms.
Grab a copy of the book "Software Tools" for Pascal or even the original in RATFOR, and port those to C. It will be easier than it sounds, since they're both, essentially, C and Unix in Pascal and RATFOR, so it's just (mostly) simple translation. When you're done with those, you end up with the basic UNIX utilities including a search utility, a sort utility, an editor, and a document formatter. Non-trivial stuff, but in the end pretty simple. And it's an achievable goal that you can just tick off going through the book.