The rafting trip on the Rio Grande is a good choice for a relaxing afternoon excursion.  The rafts are made of bamboo and carry two people on a little elevated platform at the rear of the raft.  The guide stands at the front and poles you down the river.  If you are there during the dry season (which is when most of tourists would be there), the flow is low and there are no real rapids - just a few little riffle areas that are fun to glide over - the rafts are suprisingly stable.  The river is clear and clean and you will see herons, egrets, turkey vultures, and schools of fish.  Near the launch point, you will experience the "in-stream vendors" - young boys who swim out to the raft, thrust a flower in your hand and then demand a "tip" or an enterprising fellow with a "Red Stripe" raft selling Red Stripe beer, soft drinks and even wine.  Take some small bills with you if you want to support these vendors - there were only a few of them.  There are places to stop and have a swim along the way and your guide will stop at your request.  The guidebook suggested that lunches be shared with the guide - he seemed to appreciate it (plus his tip).  The whole trip takes about 2.5 hours and is a great way to just chill out and enjoy the nature around you.  If you are there when there are no large cruise ship tours you will have the river to yourself.