i cook with cactus all the time. you can take a whole paddle, stuff it and bake or bbq it. cut it open and put in some onion, oaxaca cheese, lil cutup cactus (nopalitos), salt, pepper, bay leaf, mushrooms, etc.. then just bake it for a while or throw it on the bbq.
mostly though i cook with the nopalitos you can get in a jar. i put these in stir fries and mexican dishes like huevos mexicana to go on tortillas. a big hit is a real simple stir fry of onions, garlic, ginger, chic peas, nopalitos, soy sauce and cumin with some brown rice.
jackfruit, i dunno. always see it at the asian market but never ventured.