Seed Saving for some plants are simple. With an inbreeding plant like Tomatoes one can save a beautiful ripe fruit and save the seeds with a high likelihood of the plants from those seeds producing very similar Tomatoes. A Tomato Flower has both the male and female parts in one flower. A bee could cross-pollinate two different types of Tomatoes, but the flower is not dependent upon the bee to produce the fruit. A simple way to enhance the process of pollination with Tomatoes is to daily or occasionally shake the plant.
My favorite species to Hand-Pollinate is C. Pepo. Almost all of your favorite squashes belong to this species and could cross with each other to produce very strange combinations in the next generation. Summer Squashes like Zucchini, Yellow Crookneck Squash, Patty Pan, Cocozelle; Winter Squashes like New England Sugar Pie Pumpkin, Howden and other Jack-O-Lantern Pumpkins, Acorn Squash, Spaghetti Squash; and even Gourds! A wonderful resource that I reference frequently for classifying species and learning the details of Hand Pollination is the book Seed to Seed by Suzanne Ashworth of the Seed Saver’s Exchange.