The internal flash has 2Kbytes erase blocks, so you can reserve some of them for storing data. Just decrease the flash size in the linker script e.g. by 8 Kbytes to give you 4 blocks at the end. See flash memory organization in the reference manual, and the user manual of your linker/toolchain.
There are a few drawbacks of using the internal flash though, it has a quite limited number of erase/write cycles, and writing/erasing the flash blocks program execution for a long time, nothing (not even interrupt code) runs while a flash sector is being erased.
There are 128 bytes of backup RAM available and preserved as long as there is battery power. Is there a battery holder on your board? This might be the simplest to configure and use, and you'd get a real time clock as a bonus.
And then it's possible to hook up an external $0.20 EEPROM IC.
