Most of the airlines and hotels for that matter have shopping portals which give multiple miles or points for each dollar spent thru their portals. Delta has good partners as well such as energy, dry cleaning, wine clubs etc and occasionally has good sign up bonuses for them. The airlines have dining clubs as well such as Delta's Skymiles dining that give extra miles for each dollar spent and they often have bonuses as well.
For newbies there is still Club bing to get the 4,000 or so miles allowed.