I don't do any extra work per say.
Other than listening to her read.
But we immerse her in her subjects. So if she's doing about castles - we'll go visit warwick castle. Then if she wants to buy something from the shop I tell her how much she can spend and she needs to work out if she can afford it.
We get her to read our text messages as they come in sometimes, so if my mum texts her I'll open the phone and ask her to read it to me, and then reply for me.
Things that make her feel like she's not studying...but it's increasing her application of the skills which then spill over into her education.
And if day trips aren't possible, we'll youtube a video about it for example. So when they were do volcanoes, we watched a few videos online about them.
When they were doing geography of the UK we visited Wales for the day (I'm aware that's not necessarily everyones cup of tea, an 8 hour round trip, but we're weird)
And my DD is excelling in all areas, so (probably by luck) something is working.