Assuming you have a DP, don't stop prioritising your relationship. Make time for each other, talk about stuff other than the DC, listen to each other, don't take each other for granted, don't undermine one another in front of the kids and try not to get drawn into competitive tiredness or whose day was worse! Remember one day it will be just the two of you again.
Don't bother with bath toys, they go mouldy inside and are a pain to clean. Never met a small child who didn't have a great time with some cheap plastic cups, bowls, a sieve and a plastic spoon in the bath, zero mould, no cleaning required!
Don't take your small child's behaviour personally. They're little, they're still learning and don't know how to manage their emotions yet. It's not you, it's just the developmental phase they're in. Actually, this applies to teens too!
Mattress protectors and pillow protectors. Always. Trust me on this, you don't need to be hand-scrubbing vomit or shit out of a mattress at 3am when you can just take off a mattress protector and throw it in the washing machine.
If you have the outside space for it I strongly recommend getting a trampoline. I know, they're an eyesore and take up loads of space but my god, I'm glad we caved! Nothing has kept our DC occupied for so much time so consistently as the trampoline. It was an absolute godsend in lockdown. We can actually sit in the garden and enjoy a drink and a chat now instead of one of us always having to be running round after them.
Anytime someone offers to help, let them!! Especially during the baby and toddler years. Resist urge to say "oh, no im fine really" even if you are fine, accept the offer to hold the baby while you drink your tea, or the offer to babysit, let your DP take the kids to the park so you can get some stuff done in peace...whatever it is, accept it. You don't have to be a martyr. And if no one offers to help there's nothing wrong with asking, don't expect people to read your mind.