The poppies look to be Papaver somniferum, the opium poppy. Occurs in lots of colors from white to pink, red, purple, and almost black. The seeds can vary from pale off-white to black, with or without the blue bloom to them. The pods are smooth and round to oval. Papaver rheas, the corn poppy of European wheat fields, is commonly red, orange. or pink with elongated, hairy seed pods and stems.
Eggplant, squash, and peppers need to be fully mature. That means that squash has a hard shell (even zucchini,) peppers are fully red (or whatever their final color is--some are bred to be yellow orange or purple at maturity. There
should be no green.) Eggplant at maturity has lost its gloss and has a matte, tough, skin. The seeds should be dark and hard.
Cucumbers and summer squash are well past eating stage when ready for seed harvest. Cukes will be big, bloated, and yellow, with hard seeds. Summer squash will have a hard shell and hard seeds.
Beans are the easiest of all seeds to save, as they are both self-pollinating and easy to thresh. Lettuce is self-pollinating and has seeds with fluffy parachutes like
dandelions. When mature, then will fall into a
bucket when bent over it and shaken. There will be flowers, half-ripe, and mature seed on the same tall stalk.
Many herbs are easy to save seed. Mullein, Evening Primrose, Lemon Balm, Mint, and many others can just be cut and placed head down in a bucket to fall out as they dry.