Something traditional and lasting rather than tactical. A slip-joint pocket knife in jigged bone or stag, from a maker with a long history and a lifetime warranty, reads as a keepsake in a way a modern folder does not. Case built its reputation on exactly this kind of knife.
A retirement gift is about time, and traditional cutlery carries that association. Jigged bone and stag handles, nickel silver bolsters, and a proper walk and talk all read as heirloom rather than gear.
Case has been making these since 1889 and warrants them for the life of the owner. That combination is hard to beat for this specific gift.
A trapper or a stockman pattern is the safe traditional choice, and both are genuinely useful. If the recipient is not a knife person, a smaller pattern is friendlier than a large one.
The warranty matters here more than usual, because this is a knife somebody may still be carrying in twenty years. See the Case warranty.
Keep the box. If the recipient holds the superstition, tape a penny to it. See is it bad luck to gift a knife.
Engraving is a common request for this occasion; see engraving.