Have you tried Pilates? Those workouts can really work your core, and you dont need any equipment. Giam puts out good Pilates videos.

Get a book or DVD that show the proper way of doing situps, planks, lunges, squats, etc. Most people do these exerices wrong and get no benefit.

Still, after awhile this alone could get old and boring (particularly alone in a hotel room). Suggest that as you travel check out for local gyms in the area for day passes.

Alternate books with each set and that'll even things out. :)

you can do that very thing, FREE, here at: AirportGyms.com :)

The number of things you can do with your own body weight is only limited by your imagination. Offhand I can think of about a dozen variations of pushups, plus squats, mountain climbers, burpees, dips, ab and lower back work, and assorted plyometrics that combine a bunch of moves that will keep your heart rate elevated. Yoga move are also great, and will get your heart pumping a lot more than you would think. A couple of times a year I do workouts for 4-6 weeks that involve nothing other than moving my own body weight.

Add a couple of resistance bands and you'll practically be able to duplicate any workout you could do in a gym. Cardio's pretty easy as you just have to hit the pavement. Heck, I can hit 85% of my MHR in a 2 foot by 2 foot space, so you don't even need to go outside.