Considering there are things you do each day and / or regularly each month, there’s really nothing much more you need to do at year end and shortly after.
The areas which are mostly just addressed at year end are related to stock taking to ensure that all of your goods get counted at least once a year. For this you’d perform a full stock count as close to year end as possible to ensure that you’ve got the most accurate data within your books at the balance sheet date.
Another area companies focus on at year end relates to taking inventory on its liabilities and receivables. Similarly to counting your stock, you’d confirm your receivables and payables with third parties based on your own risk assessment. There are various possibilities, you can confirm all balances regardless of their amount (including also zero balances if there have been transactions within the period) or you can opt to confirm certain balances based on a treshold or you just confirm your top 10 customer and suppliers for instance. The method for making the selection is entirely up to you, however I would suggest consulting with your local legislation as some countries may be stricter in terms of what you’re obliged to do in terms of taking inventory of your receivables and payables.
And then there are taxes you declare once a year. That’s also something that’s done shortly after the year end mostly and include specific taxes for the whole reporting period.