The most failsafe things I grow are blueberries (I have half a dozen bushes now but have had fruit from them from the first year although it wasn't all that much.
We now get enough to stuff our faces, make jam etc... & freeze enough for the rest of the year as the bushes are bigger.
Very low maintenance.
Raspberries are prolific croppers as well which seem to increase year on year.
Strawberries - mine are in self watering towers (sadly quite expensive) & I move them into the unheated greenhouse for the winter.
I have strawberries cropping from around Easter this way through to Nov/Dec depending on the weather.
If you remember to poke the little shoots into the earth you will be able to at least double your plants for the next year & always have younger plants as the oldest ones ideally need replacing every few years.
It is worth getting a better variety than the ones you get in Wilko or Lidl sort of places.
Cambridge Favourite (type of strawberry) are one of my personal favourites.
Grapes seem to grow like weeds if you are in the warmer parts of the country.
I have a vine in the greenhouse because I get an earlier start & longer fruit producing season that way.
Don't know how they would fare in the midlands or above.
Don't just grow vegetables, fruit can make all the difference to a bland or monotonous diet.
Peppers - I grow mine inside on the kitchen windowsill then if I remember put them in the greenhouse in the summer.
Sometimes I just leave them in the house as they never get any bugs etc... if I do that.
You just have to remember to pollinate them if they are inside.
Courgettes - these are lunatic plants once they get going.
One day there is a giant yellow flower, couple of days later it is a courgette. You pick it then suddenly find there are more flowers.
It's like the more you pick the more grow.
We get to a point every year when I uproot the plants because we can't bear to eat any more courgette even in cake & nobody else will take them off you either.
I grow mine in pots as they seem to do much better a bit off the ground.
I say pots, I mean those cheap black buckets you buy for a pound or two in a diy/household sort of shop to wash the car etc...
I just poke holes in the bottom with a heated bradawl or drill a hole if the drill is out.
Leeks.
We still have leeks in the garden because there are only so many you can eat before people start complaining it's leeks again.
I must remember to go pull them, clean them & freeze them later.
Most salad is easy to grow so things like spring onions, come & cut again leaves etc... are a good idea too.
I grow a lot of other stuff but these have always seemed to be the lowest maintenance, easiest to grow to me.