1) Seat bag, several different sizes and styles available.
2) WALD rear "paperboy" baskets (the largest you can get). You can make a liner for them (or perhaps your mum or wife/girlfriend, if they sew and you don't) out of some inexpensive canvas or nylon canvas.
3) Handlebar bag, several sizes and styles available.
4) WALD handlebar basket. Paperboy extra large (most versatile) or folding. Some of the folding front baskets have optional liners.
5) Two child trailer (Walmart, etc) leaving off the seats and other child things. These make an excellent cargo trailer, with a 100 pound "official" load capacity. I use a trailer whenever I use my mountain bike and beach cruiser.
6) You have a rear rack, rear panniers are an option, or a rear trunk bag.