I work my abs every time I walk! Of course, it won't give you the same kind of workout as an ab machine, but try holding your stomach tight as you walk. I guess you could even do it sitting down, but it feels like it's working my stomach more if I'm walking. Do the thing where you pull your belly button in toward your back. Don't suck in, just make your stomach as hard as you can, like you are blocking a punch to the gut!
I also do ab exercises standing up holding free weights. There are plenty of different ways to move to work your abs. I'm not a fan of crunches, so I do "balance" type exercises using the weights and bending and twisting and all that fun stuff, LOL.
Hope this helped.