I own holiday cottages! Have a look at the Netflix programme 'Stay Here' as there are some great ideas.
A of thumb is to spend 10% of the cost of the first night's stay.
Do be aware that if you provide any alcohol then strictly speaking you should have a licence. This is obviously ignored by thousands of people, but it is the law. Visit Britain have all the rules in the Pink Book which is online. Do also be aware that to provide any food at all( even packaged) you need a basic food hygiene certificate, but your employer probably has this.
I live onsite and my accommodation is advertised as luxury.
I leave an afternoon tea in a cake box, it is homemade cakes and scones with clotted cream and jam. I also leave a homemade loaf of bread and a large bunch of flowers from my garden. Sometimes the flowers end up just foliage with just a few blooms. This is the 'surprise'.
I advertise a light breakfast for their first morning which is tea, coffee, bread, croissants, butter, jam, marmalade, milk and homemade granola.
In two years I have only had the afternoon tea not eaten twice! The thing that gets left most often is the granola- it is not even opened a lot of times.
The welcome pack nearly always gets mentioned in reviews.