In biology libraries, Nucleic Acids may be encoded as 4-bit types (see for example BioSymbols.jl). This can be used as a context for an exercise introducing primitive types, or other concepts related to bit manipulation.
The v2 exercises hamming, rna-transcription and nucleotide-count could be reused but with primitive types instead of strings/chars.
You could consider adding those exercises twice (perhaps as practice exercise) in the progression path, once with strings/chars and then later on with primitive types.