I like to keep science kits around. My kids always enjoy the thrill of opening and using them. I like having something complete to pull out when I've had too busy of a week to plan a project. My remind my relatives that they can browse Home Science Tools around holiday or birthday times if they don't know what to get for the kids.
I often find kits for a decent price at discount resellers like T.J. Maxx or Marshall's, and warehouse stores like Costco or BJ's Wholesale Club. These are hit or miss, but usually the hits are pretty good. I am already looking, buying, and putting away for Christmas.
This photo shows ds#2 with a Sensory Dome he got for his birthday, which is a terrarium with plants that appeal to the various senses. The boys had a great time putting this together, and ds#2 in particular took keen interest in reading about the plant information on the planting tags.