Best things I did:
Read to them endlessly, and sang to them.
Talked to them all the time and didn't modify my vocabulary just because they were small children.
Stuck to the non-negotiables (eating only at the table, going to bed and staying there unless ill), but didn't worry about the small things (going to bed in wellies and fairy wings was fine, so long as they stayed in bed).
Had a good routine - and not just a sleeping/eating routine (which we did have), but a weekly routine. We didn't stick to it slavishly but it was based around (e.g.) Monday being toddler group, Tuesday the day we met friend X, etc, etc. I think it made them feel secure.
We did lots and lots and lots of walking around our neighbourhood, so they would chat to all the neighbours, the butcher, the lady in the post office etc.
And this is a really big one: there were no smartphones, ipads etc. So if we wanted to go to restaurants, cafes etc, we had to be fully engaged with keeping the DC occupied with hangman, Old Maid, I Spy etc. This is much harder, but infinitely more rewarding for children and parents alike. It's easy for me to say "I'd never stick my child in front of a screen", but it was not an option then.
I suppose these all add up to "spent all my time with them".
Things I did wrong:
Didn't get them to do anywhere near enough domestically, the result being that they now act as if they've been asked to walk to the moon if I ask them to put a cup in the dishwasher.