Hmm ... at 7 months .. she probably would just enjoy changes of scenery, like the other response ... just taking a walk outside. Rotate her toys in and out - so she doesn't get bored with the ones she has.
I also think that at 7 months ... mental stimulation doesn't require ... going to big expensive places and buying fancy toys .. there's YEARS of that ahead of you ... now is a perfect time for simple things as everything is new for her.
I pulled this from another site for 7 month olds:
Play peekaboo. When you reappear, your baby's delighted laugh signals both his enjoyment of the game and his sense of relief. He's just beginning to learn that you're still there, even if he can't see you.
Let your baby help you turn off the light switch while you say "lights out." After a while he'll make the connection between the light being switched off and the room getting dark.
Give your baby toys to play with that can be named easily: cup, telephone, doll, kitten, spoon, dog, block, rattle, banana or clown. Name each toy as he reaches for it.
Have a play date with a baby your child's own age. Your baby will also enjoy watching older children play.
Cut out large, colorful pictures from magazines: a telephone, a dog, an airplane, a spoon, a teddy bear. Then paste the pictures into a blank notebook. Sit your baby on your lap and "read" the pictures together.
Try dropping games. Show him how to drop a hard ball into a large plastic bowl; he'll be intrigued and want to try it on his own.