What kind of exercise to start with if I haven't really been active for several months?

I would start with the easy stuff that you say feels like it might be wasting your time. It isn’t. And before you know it, you will be pushing yourself a bit more. The important thing is to do some exercise regularly every day. Walking is also a good way to build up strength and stamina.

Swimming is also excellent for low impact which will not put undue strain on joints or muscles.

Walking a bit farther and faster every day. Before you know it you will be ready to combine walking and jogging, and then jogging by itself.

Yep I agree with the walking and jogging. If you go to this site you will find lots of information on beginners walk/jog/run programs.

Exercise balls are good too and most come with a dvd or a chart that will give you the basic exercises.

Yes, walking is easiest to start.
Walking the stairs up and down.
Cycling, swimming, running, jumping, etc.

Indian exercises:
Pranayam, four steps- 1. Take the breath in slowly or inhale slowly, 2. Hold your breath, 3. Slowly push your breath out or slowly exhale, 4. Hold your breath, repeat this cycle for 5–10 minutes at least. Increase the times in each step as far as you can.
Surya Namaskar, in which Pranayam is built in.
For Yoga, you’ll require time and a good teacher.

Other things include, take right food in right quantity at right time. Take enough sleep.
Drink enough water. We drink here Lemon juice/syrup to avoid de-hydration and freshen up yourself; take a glass of water, squeeze the lemon juice in water, add 1 or 2 spoons of sugar, ¼ to ½ spoon salt, stir well and drink.

interval training is a great way to stay challenged at any level. You can do this indoors (like on a treadmill) or outdoors. Start out with a 2–3 minute walk followed by a 1 minute light run. Repeat for as many rounds as it takes to complete a workout session. Next time try making one of the intervals a bit more challenging by resting one less minute or by extending the run by one more minute. Continue to make the intervals more challenging each time you go out.
Adding just one minute of intensity makes it an attainable goal, yet can make enough of a difference in the workout to ensure that you are progressing.

Run!! Its as simple as that, do interval running at first to get your body used to exercise again

Try a 2 minute run with a 1 minute walk at first for about ½ hour and i’m sure in no time you’ll be able to run without any intervals.

Running is probably the best exercise over all and its the best way to build up your stamina fast