It's hard to have too many sewing notions. I always make sure I have a packet of hook-and-eye fastenings, metal snaps and safety pins in my sewing box. I bought a button assortment and keep those too, along with any extra buttons that come with a garment I bought.
It's not a bad idea to have a couple of packets of elastic, either, in 1/2 inch and 1-inch widths. These can be really helpful for a mending/restructuring project. Elastic can deteriorate over time, so I always keep some around. You never know when you might need it!