Radish and spinach don't normally cross with other plants but might if they have close relatives nearby. ie: if you have a couple of types of radish growing next to each other they might hybridise. Different types of melon will also hybridise so if you want seed then try to only keep one species/variety of each plant.
Tomatoes can be left on the plant until they start to rot. Then take them off and put them in a sieve and squish them up and rinse them off. Separate the seeds form the pulp and dry them out.
Chillis & peppers can be left on the plant until they start to dry out and shrivel up. You can take seeds from them when they go red and they are usually fine too. I just prefer to take them when they are really old and withered. Then cut the pepper open and scrape the seeds out and allow to dry.
Sunflower seed can be collected when the flower head starts to dry out and die. Then just put the flower head in a paper back and leave for a few weeks. Shake the bag occasionally and the seed should fall out of the flower head. They can then be stored or shelled and eaten.
Stuff like radish, lettuce, spinach, etc will go to seed at certain times of the year. They send up a flower stalk that develops a seed head. Once the seed head has started to dry out and the plant starts dieing you can collect it and put it in a paper bag. Leave it for a few weeks and then shake it to separate the seeds from the pods.
I sometimes collect Kangaroo paw seeds. They are native to WA and produce a seed pod that dries out before splitting open and scattering the seeds everywhere. You have to get the pods at the right time otherwise there is no seed left. I also collect orchids seed pods and store them in small paper envelopes. The pods dry out and the spore falls out of the pods into the envelope.
If I manage to collect enough Wattle (Acacia) seeds then I sometimes use them to make a type of wattle coffee. It tastes more like nuts rather than coffee and has no caffeine.
