Here where I am, taters do good, lettuce and some greens also. Tomatoes are ok. But the biggest winner in my experience, hands down, are the cruciferous veggies, the cabbages, brussel sprouts, broccoli, kale. They all grow like weeds!
And the slugs pretty much ignore them.
We grew some great tomatoes this year. How do you let the remaining tomatoes go to seed? Keep them on the vine? or scoop out the seeds and dry them somehow?
One thing you discover is for some plants, one of the best fertilizers and growth factors is the fruit itself. Tomatoes are like that. So are the gourds, like the pumpkins.
If you want to have pumpkin plants next year, simply smash up a pumpkin in your yard, garden, compost pile! Leave it there all smashed up, maybe work it into the ground a bit.
My biggest and best tomatoes this year are growing in my compost pile. They are all rogue plants that just took off from the seeds in there! My compost pile is probably 90% coffee grounds.
To save the seeds, you can scoop them out and dry them like on a paper towel. Then, after they are dry, save them in envelopes, write the plant type and year on the envelope.
I must have close to a five gallon buckets worth of various seeds from previous years in envelopes.
Just keep them cool and dry and in the dark and they should do fine when you replant them.
Note that some seeds do best if you soak them in water for like say 24 hours before you plant them. Peas like that.
My best advice is to play around with it! Have fun and see what works! And enjoy your harvest!