Could you try an exersauceror a jumperoo? These things are for babies who are a little bigger and worked great for Cate!!

How about a walker? they can sti in it and zoom all around and my 2 year old son sits in Sister's so it can hold lots of weight.

I was going to suggest a high chair if you have one that reclines and is height adjustable but that wouldn't be too convenient to take to your friends place... I have seen baby chairs at target, that are like a rocker but are somehow adjustable and can be used for a baby all the way up to a toddler... might be worth a look.