Alfalfa cubes are too high in calcium and will cause calcium buildup in the urinary tract, causing serious problems down the road. But, this is generally only a problem if you feed them on a regular basis. I would suggest buying a bale of good quality grass hay, prairie is what I'd recommend, either that or brome or timothy. A bale of high quality prairie hay costs me $3 and lasts...usually around a month. My buns are spoiled on fresh grass, they tend to snub their noses at 'that dry stuff'.
I do feed alfalfa as a treat, however. My young bunnies get 1 cube to chew on per week, my older bunnies the same. I also let my bunnies 'free range' as much as possible. I keep my youngsters, not old enough to breed or weanlings, in my chicken tractor. There they can get up and out of the weather, or go down and eat fresh grass. My older breeding buns get a nice big cage, I'm planning on modifying it this week so they can better get to the grass. Usually the cubes go untouched, but it gives them something to grind their teeth on if they want it.
For treats, try melon rinds. My buns just about maul me when I come with melon rinds. Or apples, pears, that sort of thing. I usually give them whatever veggie scraps I have too. They die and go to heaven over cucumber peelings