Can't get near a pool or gym? No problem. The year I attempted/finished Ironman Hawaii I worked in 40 cities across the North American continent in the four months leading up to the race. Some weeks...four new gyms I had never been in (and probably will never be in again). I met tons of great folks from all over North America and from all kinds of countries and backgrounds. I know this didn't help my overall time in Kona but I wouldn't trade the experience I got for a slightly faster time.
I have two types of tubes I use. One looks like a figure 8 and can do a number of different back/chest work as well as glute work- even in a hotel room. The second tube I have is a tube with two handles on each side. This is great for everything from squats, chest, curls, abs and even swimming with some creative knotting. If you don't have a tube you can do isometric work with a towel if you are really desperate. My favorite workout is to start with a short tube (high intensity) and do 5 reps, then go to a mid-length grip and do 12 reps, and finally a long grip and do 20+ reps. Do this with 30 seconds rest between sets and I guarantee you'll be sweaty and tired very quickly.