You could conceivably open a few accounts. For example, a bank account and a credit card account. Then the accounts will be older when evaluated for credit when you return. This would look better than opening fresh accounts later. But don't expect a big difference in score. And you'll be stuck with those accounts in the future, otherwise you lose the benefit. I wouldn't worry about maintaining balances now. You can wait until you come back. Occasional purchases may be helpful.
What they really want to see is a regular and sustained use of accounts without missing payments or overextending. But if you're not going to be here, you can't really do that. Note that good credit scores are based on seven years of data, preferably a lot of it. Opening a few accounts can't substitute for that, even if you put balances on them.
If you're not here, you won't be paying rent or utilities. You won't have a proven payment history on the most common accounts.
If money were no object, you could do something like purchase a house or condo that you could rent out, utilities included. That would build up a payment history. But if money were no object, you probably wouldn't be worried about your credit score. It's more practical to just live normally and be sure that you always live within your means so that you don't experience negative credit events.
You might think about why you want a good credit score. Is it to borrow a lot of money? You might be able to spend money to achieve that. Is it to save money on future borrowing? If it costs money now, how much will you save total?
Opening accounts now that you won't really use until you return is about the only thing that you can do that won't cost you money. Perhaps put a balance on the bank account--at least you'll get that money back some day. Maintaining a balance on the credit cards would cost you money in interest charges, and you don't really benefit from an improved credit score until you use your credit. So the interest fees aren't really buying you anything.
