G.B.
Wal-Mart hs nearly a whole row of infant toys in the toy department. Any of them would be fine. Also they have rows and rows of stuff in the infant department. Give him a tippy/sippy cup, his own dish set and flatware too, stacking toys, what about something from Babies R Us, soft books, bath books, now that he's sitting up better what about a bathtub ring where he can sit up, there are so many things that at 6 months he will be growing into soon.
My favorite web sites:
Check out the different catagories.
http://familyfun.go.com/
I get weekly/monthly emails that have developmental milestones and what to expect your child to start doing next. They also have a section close to the bottom on baby gifts during the first year.
http://www.babycenter.com/baby
http://www.babycenter.com/baby-gear-for-the-first-year