K.P.
My son is the same age and gets really wiggly after 20 minutes or so of being confined. I think it's partially the age- discovering what reactions they can achieve from their actions. I would suggest avoiding taking him places where he can't walk around a little. In the grocery store, I let my son help me take things off the shelf and cross items off the list.
When I have to shop for clothing or gifts, I have a shopping "agenda"- literally. I know which stores to go to first b/c they will be very boring for him and which items on the list are somewhat optional or can be done another day- those are always last! Before we go out, I make a list of what I need and where I can likely find it. That way we spend less time browsing and more time chatting and getting through the errands.
I never shop for anything special or important with my son in-tow! He hates having me shop for work clothes and it ends up with us both upset in the end.