Jump rope.  You can do it indoors and it gives you an opportunity to revisit the classics, such as:

I know something,
But I won’t tell.
Three little monkeys,
in a peanut shell.
One can read,
And one can dance,
And one has a hole,
in the seat of his pants!

Jumping rope, a perennial workout of boxers, improves coordination, agility, quickness, footwork and endurance.

It’s dead cheap, as you can probably find a piece of cord around the house that you can use to get started, and the most expensive high tech jump rope probably isn’t more than $25.  I got one recently from this place for $15, shipping included.

With practice, you can learn to jump like this or this.  According to the step equivalents, 30 minutes of jumping rope is equivalent to 9091 steps.  Not too shabby!

P.S.  I took my own advice.  Let me tell you that it is challenging and exhausting, but a great workout!