I use an older bike though while very functional is seriously in need of a paint job,new bar tape and showing hints of rust.In place of panniers I bungey a duffel to the rear rack that is easy for me to remove but complicated for someone else to figure out.I also attached a daypack above this in a similar manner that I sometimes detach and wear into the store or on dayhikes.About the only valuable I carry off the bike besides this is my pump but if I did carry a computer I would definitely carry it into the store in the daypack.

With all of this being very functional but not fashionable, I just lock the bike to a rack, sign post,tree or whatever and never worry about it while in stores etc.