What a nice idea. I'd think about the things that the grandpa enjoys, and will probably want to do with his new grandchild, like any hobbies or interests. Fishing, woodwork, travelling, that kind of thing, then take the theme from there. Add a plaque cookie that says "Congratulations Grandpa!"
Otherwise, a flower bouquet always works well, in 'manly' colours. Years ago I was away from home and sent my dad a real flower bouquet (oranges and browns) for his birthday. My mum said he'd never been so surprised or pleased, and since then I've never had the worry that 'men don't like flowers' (except my husband, who has hay fever!!).
Do they know the sex of the baby? Does the grandpa have any hobbies? Into sports? I think it would be neat if you could take the grandpa's interest and make it for him and the child. Hope that makes sense!
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