My 5-month-old fusses when we don't bring her out so we we end up going to places like the mall, the park, and the zoo everyday. Even the supermarket seems to delight her. I wonder though - is there such a thing as too much simulation for a young baby?

I don't think that there is such thing as over stimulation. You don't have to bend over backwards just to please your baby. Remember you are the boss and they should know that so they won't push you over as they grow up. If baby is fussy, does not mean that she likes to go out all the time. Try having a regular play time with her, invite other babies and have playdates, take her to the void deck or somewhere near.