Anything you like and can pay for. You can see what you've bought by looking in the item in your inventory that looks like a picnic basket; it's labeled "Souvenirs."
As the game goes on, you'll find there are two things that you need to buy here, so you might want to glance through the selections just to see what's for sale. Most of the items are just fun gewgaws, but you'll need a couple of them as the case progresses -- those batteries, for instance.
After you've gotten a call from home, you'll think of one more thing you need --
Hint 4
-- a present for poor long-suffering Ned. When you buy something you want to give him, click on the wrapping paper to have it giftwrapped. You can only wrap one item, so choose carefully!
Buy me a coffee