a good substitute for pineapple when cooking a ham

Pineapple brings a touch of acidity, sweetness, and general fruitiness. I am going to assume you would have been using canned pineapple, so the enzymatic action is not really a factor (and it would be stopped as part of the cooking process). It is also hearty enough in texture to stand up to the baking.

For the juice, I would recommend basic orange juice, perhaps cut with a little extra lime juice for complexity and acidity. For a fruit garnish, peaches (frozen or canned since it is not peach season) should work quite well.

If you can find good mango, that would also work very well.

This application is very flexible and forgiving. With the soda, and the cherries, you could even omit the pineapple part entirely, and still have a good outcome.

Based on my experiences with sweet-and-sour asian dishes, I'd say good or even okay mango would work just as well, if not be an improvement. (I find canned pineapple or pineapple juice to be cloying or bland compared to fresh, and generally like mango better in savoury foods.) If you're going to puree / juice, and not eat it as-is, you can "fix" it not being perfectly ripe with some lime juice and/or brown sugar or honey. (In fact you might want to do those even with a ripe mango - mango/lime/honey is a delicious combination in and of itself.)

Lot of people are talking about the acid in the pineapple. That's irrelevant in this case: pineapple runs around ph 3.5, and most soda runs ph 3 or less (coke runs around 2.5!). Additionally, a ham will be so heavily processed that the usual "It'll help tenderize the meat!" bit won't apply either.

So really, it's a question of flavour, and that opens the doors for a wide realm of experimentation. Orange juice, mango juice, some kind of fruit syrup from your kids favourite fruit cups...Sky's the limit.

My family always went with the Coca-Cola ham, rather than the more educated Dr. Pepper version. The Coke ham is basically ham, basted with a mix of coke and nasty yellow mustard (French's or similar). We studded it with cloves, rather than cherries, so it's a bit more savoury.

Hard as it may be to believe, the taste is quite good (hell of a lot better if you go out and find coke with actual sugar which IMHO makes for a better glaze). This would probably be my choice: I'd rather do a slightly different dish than to monkey around with substitutions for integral ingredients.