I'm sure other folks will have some good ideas re: cash back, but I tend to focus on miles. One thing you could do (if Southwest is useful for you) is get a Companion Pass, which lets someone travel free with you on Southwest for the rest of one year and all of the next (even when booking with miles). You could get that with $10K in spending.
To do this, you'd open a personal and a business card (both with 50K signing bonuses). Once you hit 110K (which you would after spending $10K split between the two cards) you'd get a Companion Pass which, combined with the 110K miles, would get you a bunch of free flights. The annual fee would be $168 ($69 for personal, $99 for business) and you might need a referral to get the 50K bonus on the personal card (I did).