I've used the past week to work on something that should have almost no visible impact for my customers.
It's a series of infrastructure improvements.
For me that involves software upgrades and rewriting some code.
So far everything has gone well, which means that nothing looks different to customers.
Sounds like a waste of time when phrased like that...
But these sorts of improvements are needed in order to continue to operate smoothly.
By taking the time now, I'll be able to take advantage of them later this year on visible things for customers.
For your store, you probably have similar infrastructure improvements that you'd like to get done but haven't gotten around to yet.
They could be app changes, process improvements, or even something as simple as organizing your warehouse.
While your customers won't see the impact of those improvements now, they too could become visible later on.
My advice is to make sure you budget some time for regular improvements and cleanup. Delaying them too long could end up costing you more in the long-run.
My normal strategy is to cycle between visible improvements and invisible improvements. Say a couple of weeks for new features on my apps, followed by a week of infrastructure, and then back to more visible features.
You could use the same strategy or allocate a few weeks throughout the year. A post-holiday fix-up period seems to be common with some stores.
One simple improvement you can make is to get your structured data in order.
Google has been working on Rich Snippets so I bet there's going to be more search enhancements coming out in 2018 for stores with structured data.
And JSON-LD for SEO is the easiest and safest option in the Shopify app store to automatically add that data.
Already trusted by over two thousand stores, it's among the top SEO apps overall.