Would doing just these be good enough for a workout, and also how many of each?

[DelayedMist]

Cardio: Running/Tennis/Brisk Walking/Sport

Muscle Strengthening: Push - ups, Weights and Squats

Stretches

[WindAtMyBack]

Depends on what you want to accomplish.

The cardio looks pretty good. "Sport" can mean anything so it's hard to comment on that. Runny and tennis are high energy, especially tennis. You can do regular relaxed walking instead of brisk walking. It's still very good for you.

For muscles, I don't like weights as the main source. I think it should only be a supplement to more natural ways to gain muscle and strength. I like body weight exercises, but you need more than just pushups. Do a variety of pushup styles along with other body weight exercises.

Do you have access to any gymnastic equipment? Rings, Parallel bars and pommel horse are all excellent workouts. Plus it's great for balance, which shouldn't be neglected.

Do you have a bicycle? It's good for cardio and leg strength. Just change gears to switch from one to the other.

Stretches - yes. Make sure you're warm and don't pull anything. Do buddy stretches if you can. If you do toe touches, be careful with your form. Stretch the back of the legs, and maybe a little back muscles, but don't put a lot of pressure on your spine.

All in all, what you listed is pretty decent. Just make sure to get a variety. It's a heck of a lot better than sitting on your ass all day. :)