I am a big believer in varying exercise for a few reasons:
* To burn more calories as different exercises burn different amounts of calories
* To keep things interesting and fun
* To work different muscles
* To keep your body on its toes so to speak as it can tend to get too comfortable with the same thing
The walking is great and early morning exercise really sets you up for the day, the Zumba is also great and i'll bet it is fun too!
A few low cost suggestions are:
* Skipping - this burns stacks of calories and gets the heart rate right up. There are many variations of skipping so you don't get board. A skipping rope will cost you around $10 and you can get an excellent workout done in just 10 minutes. You can youtube 'skipping exercises' and there are a lot of them out there so this will keep you aiming towards being able to do certain skips. At first you will probably find that 1-2 minutes has you pretty buggered, that's fine, just go for as long as you can, then give yourself a 30sec break but keep moving, so maybe doing half push ups, sit ups, light jogging or even marching on the spot, then when you catch your breath, go again. Doing this for just 10 mins to begin with and you will be surprised just how hard it is! Skipping is a full body workout. You can expect to feel the effects in your core mainly, but also legs and arms.
* Workout DVD's. Have a search for these on ebay. Find ones that you think you will enjoy, some suggestions are dance, kickboxing, cardio etc. If you spend some time searching you will find some for as little as $10 (especially if you buy from overseas).
* You can get a good home workout costing you not a cent. An example would be:
Warm up for a few minutes with a jog around the block or a few minutes skipping
20 push ups
20 mountain climbs
10 jump squats
20 star jumps
30 knee lifts
Do these exercises 2-3 times then do another jog or skip and then stretch for a cool down. Go hard at it, it will only take 10-20mins depending on your fitness level and for how long you warm up. Guarantee to work up a sweat if you go hard, nice short workout, perfect if you don't have much time and best of all it costs you nothing!