pje120

And you can't make a decision once and all of a sudden up your motivation by 50 "points". You have to do it often, and consistently. Starting small, just like you start with small weights.

If he wants to start small it's up to him.

You can do something like this:
DayA: push ups, chair dips
DayB: chinups

3 sets to failure ~10 reps, week plan: day A, day B, rest, day A, day B, rest, rest (repeat).

Pretty basic but there isn't much to do without equipment. You can throw in some more exercises from that thread Rodney posted, but if you throw in too much you should change to 3 workout days a week (no repeated days in a week) so that you don't overtrain.

You can throw on a backpack with heavy stuff in to weigh down the exercises. This is very important: a gradual progression of weight. Don't do exercises that you can do very many reps with, they're just a waste of time unless you are training for endurance for that muscle.

Admittedly I really doubt you'll build any noticable muscle without weights, because probably the most basic principle of building muscle/strength is to gradually increase the weight... which is hard to do without weights. You'll gain muscle endurance training without using some sort of variable resistance but not size or strength. And it won't make you fitter, either. If you want to be better at the activities you listed, run.

But play around without weights for a while. I think you'll find you will be disappointed with what you can achieve without weights, and realise that training is actually pretty fun and satisfying, and find time to take the next step and get to a gym