Satellite work can be done with very little equipment and almost no weight. In fact, if you bring an HT with you a simple, very lightweight antenna is all you need to successfully work many of the low Earth orbiting (LEO) satellites now available. During Field Day at our local club my friend (and Elmer) Dave slipped outside for a few minutes while everybody else was chatting and worked a QSO or two on the satellites for score. All he used was an HT and a hand-held antenna.
A great place to get started with this is the AMSAT (Radio Amateur Satellite Corporation) web site. There you can learn about the various satellites now in orbit and available along with some tips about how to use them. I don't know which radio or antenna he used for this, but I own the Arrow II Satellite antenna with the split boom and duplexer. This antenna breaks down to a very small collection of parts that slip nicely into a custom bag. The total weight is under 2 pounds.
You will need to know when the interesting satellites pass over you in order to work them. You have plenty of options for this but allow me to give a plug for a product that I use called MacDoppler. (Note that AMSAT members can register this software at a discount.) Just to give you a taste, check out this screen shot from the Dog Park Software web site:
The point of this is simple: traveling light doesn't mean doing only limited activities. In fact, this might be a great time to try something new and different that you wouldn't be tempted to do at home. Also consider the point that even well-traveled places not sought after on HF might be very much in vogue as a satellite QSO. You could hand out a new one on a new band/mode with just a couple of pounds of equipment and a little enthusiasm. Who knows? You might have some fun, too! So, there's something to think about when you're selecting that magic 100 pounds worth of gizmos.