We took our DS when he was 7 months.
He was new to the standard booster but with an extra blanket folded up behind him (or coat in a pinch) he did fine.
I also found the multi-color links (ring like toys that link together) were great for hooking on the booster so he could play without Mom and Dad having to pick up toys every two seconds.
There is enough stuff to carry around with a little one I can't imagine adding more.
We were allowed to bring our stoller into any resturaunt we asked to and Baby Care Center all seemed to have regular high chairs, with high backs and tray tables.