We just managed to fill 3 days with a small pack of orbiezz (£2 of ebay). We started off pushing plastic anamals in them in a big mixing bowl, then tipped them in an empty bath and the kids though and kicked them around (there light but bounce), then we put some water in and splash (just made the water have bright coloured bits).
If you also have older kids, my 4yo enjoyed splitting them into colours, which we put in a jar in strips. We have put some bean seeds in at the side, so (hopefully) we can watch them germinate.
In total we managed about 8 separate play sessions, spaced out over a few days. Unfortunately I think a lot of that was novelty value, so will probably leave it a few months before getting more, but it filled some time. Would definitely recommend, I was amazed how much play my 18 month and 4yo both got out of them.