A far more profitable venue is manufactured spending with credit cards. Basically involves purchasing near-cash equivalents with a credit card that are considered ordinary purchases, then converting them into something that can be used to pay off the card in the grace period, allowing you to rack up rewards.
For instance, I purchase $2000 Visa Vanilla Prepaid Gift Cards at CVS with a card that gives 5% back at drug stores. Each $500 card carries a $4.95 fee. I then purchase 2 $999.30 money orders at Walmart using the cards (minus a $0.70 fee for each money order) using the gift cards. I deposit the money order in my bank account to pay off the card. $100 in cash back rewards- $19.80 in giftcard fees - $1.40 for money orders= $78.80 returns on an interest-free loan of $2000= 3.94% returns per go- way more profitable than any guaranteed investment is going to give you in a year.
Obviously there's a hassle in the hustling, the costs of your time, etc., but if you already find yourself in CVS and Walmart semi-regularly the marginal cost of this is basically zero. A couple grand a month isn't going to attract the ire of card companies.