If you’re walking any distance, bring a buggy so you have somewhere for the preschooler to sit and rest. Do you have a double, or a pram that can convert? It can be easier to lift out the baby, and strap in the heavier one than cajole an exhausted pre schooler home.
If you're travelling by car, always strap the older child in first. Babies can’t run away! Sometimes our instinct is to look after the smallest one first but this is one case where it’s the other way round.
Before, or on the way to the outing, tell the story of what you’re going to do, how nicely she’ll behave in precise detail (holding your hand, etc) and how pleased you’re going to be about it. Tell the story of how good she was on the way home too, it’s a great way to communicate your expectations and children thrive on praise.
We always played the high five game out and about: if I held up my palm, they had to run over and high five me. Sometimes I gave a choc button, but also lots of giggles and praise. It was a foolproof way of getting them to come back from a distance.
Activities need to be on your terms! Scavenger hunting is a great activity - you say “find me a leaf” and they run off and come back, doing much more exercise than you do. Count how long it takes to run to a bench and back and then have them try and beat their time. These are great activities when you need to stop and deal with the little one.
A sling for the younger one can be a game changer if you haven’t tried that. You can breast feed discreetly in one- even walking about.
You do have to grow a thick skin to a bit of whining. If you have to give one your attention, the other will whinge. That’s ok, especially when you need to prioritise safety,
Give a 5 minute warning but phrase it positively “we have 5 more minutes so go play on your favourite things”. And when it’s time to go, say “have a go on your last thing and meet me at the gate” and start to pack up and walk away. Dc are primed to want your attention, and will follow.
Give a drink when they’re in the car, or strapped in the buggy heading home.
Bring snacks, water, etc so you don’t set precedents of buying sweets/icecreams. By all means go out for those things but make an occasion of it, or you’ll be battling inflated expectations every time you leave the house!
Finally (I should have said this first), get everyone dressed when they get up, yourself included, so that going out isn’t a mountain to climb. It can be such a sanity saver to just head outdoors for a while and run off some energy. But if you have to get everyone dressed first, it will defeat you on those days when you just need a quick fix.
Hope there’s something useful in all that.