Hi,
I would suggest a bumbo seat - it helps your lo sit independently (in an ergonomic mold for them to fit into) and they have a tray that you can put on it so they can play with toys and feel independent sitting on their own. We do that once a day with our 4.5 month old. Exersaucers and jumparoos are also fun, but of course all of these are pricey items. Otherwise, we read books, offer fabrics or materials with various textures to stimulate their senses. We listen to music with him, and lay him on our laps and bounce lightly to the beat, etc. We put him in front of the mirror, and he likes that. Just a few ideas, I hope that helps a little...
katemom