What you need is variable levels of resistance to build strength. How you get it depends on you and your available resources. A fully stocked gym is optimal. However, if you do not have access to a gym due to travel or funds, progressive calisthenics is a complete replacement.

It's known by other names such as Convict Conditioning, Street Workout, Rough Strength, Beast Skills, Body Weight Strength Training, etc. These modern strength calisthenics will teach you clever exercises and strategies for varying resistance (points of contact, increasing or decreasing leverage, etc.).

Modern strength calisthenics have been updated from the 50's and they've put in the different levels you need and incorporated bits of power yoga and inversions, low-level gymnastics, and some martial arts strength training components. You will not be bored.

The methodologies will get you to incredible levels of functional strength, and your bones, tendons, and ligaments will also get very, very strong. You may actually slim down. Unlikely you will look like a huge bodybuilder, but you can be trim and deceptively strong. One-leg (pistol) squats and one-arm pull-ups? It is possible to get to such levels if you want to get that far.

I've been doing this for nearly 3 years. Before I went to China for work, I was searching for just such systems, and only recently it seems have they been available. By the way, I have been lifting for 30 years and was always frustrated when I could not get to a gym.

Street Workouts can be done in calisthenics parks (very common in Asia and Europe) or at home or in a private office. I look around my environment for places to practice, so it's a mental game too.

The main problem is finding somewhere to do pull-ups. However, you can do these on a bar, scaffolding, a tennis court fence, a soccer goal net frame, a tree, a wall, or even a braced sturdy door. (Wear gloves if your using walls or doors.)

As you get older, you may also find strength calisthenics to be easier on your joints, if you pick the right progression level for your current state of fitness. (You can always regress to a lower level if necessary.)

Lastly, if you are traveling, hotel swimming pools and oceans are a great alternative resistance workout. Swimming does works everything, including muscular strength and muscular endurance. Try to switch up your strokes (Day 1 Breaststroke/Freestyle, Day 2 Backstroke/Sidestroke, etc.) to work both anterior and posterior muscle groups.

Good luck, gentlemen, and keep lifting!

Recommended books (and videos)... Convict Conditioning by Paul Wade, Street Workout by Al & Danny Kavadlo, and Bodyweight Strength Training Anatomy by Bret Contreras. CC has 10 levels (progressions) for each of the six major exercises: pushups (or dips), pull-ups, squats, abdominal, bridging (back), and handstand pushups (a military press or overhead press equivalent). SW is a pleasure to read and has lots of great pictures. BWSTA will give you the variety you crave and will stimulate your thinking.

CC has the best breakdown levels I've ever seen. From super easy (rehab) to nearly impossible. SW is probably the most friendly & approachable book, and BWSTA will fill in any weaknesses from the other two. Highly recommend you look for YouTube or the professional videos from the CC people and from Al & Danny Kavadlo. Dominik Sky also has some great videos on YouTube and his site, especially on handstand pushup progressions (or, you can look up yoga inversions done by hottie Tara Stiles).

Addendum: As to the controversy of training with weights vs. calisthenics, it's a worthy discussion. Both sides can do things that the other cannot. I don't know of any power lifters that can do the harder strength calisthenics levels, but such people do exist. On the other hand, I've seen some huge gym rats that could probably bench press a small car. Strength is what we are after. Your method up the mountain can vary. (Hypertrophy vs. pure strength is another useful tangent to consider.)