The classic response would be to check out any of the 3 amigo's (Fa 31, Fa43 and Fa77) or the DA limited range such as the DA 40 or DA70 depending on how much environment you want to include in your portrait of the cashier, etc.
Esrandall makes some great suggestions for a zoom, or less expensive DA lenses, and older MF lenses would be ideal if budget is tight and you have good enough eyesight.
The Pentax 16-50 and 50-135 make a great pair for general and portrait photography on APSC, but again, these are more expensive
There are a great deal of choices, but it is all dependent on budget and taste. Ultimately only you can decide (or you can be like me and just eventually buy almost everything, still trying to decide
)