A good, simple exercise routine?!

I'm 23, around 5'2 and weigh around 110. I do not want to lose weight, but actually gain some if I can. I should also mention that my diet isn't bad, since I know exercise and diet go hand in hand. But my main concern is exercise. I am not getting any right now and I don't want to join a gym or buy workout videos. I also very easily become unmotivated when routines are too complex and you have to remember a bunch of steps (like "sets of reps"). I want to feel and become healthier and my stomach/lower abs are getting a little flabby so I want to target those, while also getting a good cardio.

So this is what I came up with:

10 minutes on an exercise bike as a warm up
10 minutes of bicycle crunches to target the lower body
10 minutes on the exercise bike again to wrap things up and end with cardio.

So a total of 30 minutes of exercise every day.

I was thinking of also climbing up and down the stairs in between the warm up and the bicycle crunches (like 5 min of stairs and 5 min of crunches), but I read that it can be bad on the knees in the long run and, although I don't have it right now, arthritis runs in my family.

Now, as I already implied, exercise is not my field of expertise (not even close) so if what I wrote above as my plan is beyond dumb, then please feel free to make suggestions, but please be nice about it...thanks :)