KITTYS!
Well, it all depends. How big are the crabapples? The kind I have are little red things that look like cherries; not much flesh, so juicing is the best option. I have had larger crabapples which look like small red apples.
Those you can make into crabapple butter--and, of course, juice. Once juiced, it can be made into jelly or the juice can be used by itself (I have used it in smoothies). And, don't quote me on this, but I think you might be able to extract pectin from them; this is what happens when I spread word-of-mouth rumours rather than check sources.
Bonus tip for the devoted is to use a food mill (I use the
Foley) to sift usable pulp from the fibrous refuse. This pulp can be used in baking in place of applesauce (assuming we're still speaking of crabapples), and perhaps crabapple butter. Lots of options depending on how much effort you want to give it.
Nice, nice, nice! Was this at your grandparents' place? It seems like you mentioned they grew quite a bit of fruit. Grape jelly, certainly, or canning the juice as is works, too. Add a quart jar of juice to a gallon pitcher, fill with water, add stevia or sugar to taste, and you're off to the races.