the biggest one possible. Once they start wriggling those little ones are almost useless.
scope out Deals Direct - i got a huge one (prolly similar to a few together), has a water play mat (so mat with water and floaties in it) at one end, tunnel with toys in the middle, and touch mat at the other with light up things and music. even with delivery it was less than $90.folds up into a bag for transport so we'llbe able to take it other places as well as at home.

DH also grabbed one from Big W last week for $98 - is in the catalogue - looks like a little box with two arches over it but the mat bit has heaps of different ways to fold it for different activities...