Inner tubes are a lot like condoms--they're both made of rubber, and should be chosen based on enjoyment, purpose and size. Before heading out to the river, you may want to try one on--the inner tube, that is. Be sure to purchase at least one spare tube especially for your beer cooler!
Inner tubing is the poor man's rafting. More importantly, it's an activity accessible even by the most inept of athletes. The basic instructions are as follows: 1) Sit in the tube--once it's in the water, dummy. 2) Let the river's current take care of the rest.
Some of the state's best downriver floats are within 20 minutes of Portland. On the Sandy River, start at Dodge Park. Exit 12 from I-205, hwy 212 through Boring to Ten Cyck Road; follow signs to park. (Also look for the rope swing!) Get out at Lewis & Clark State Park.
To return to your launching spot, you will either need to hitchhike back, or pre-plan and park a car where you plan to leave the river. (Remember, park the second car DOWNriver, as you will float with the current. You wouldn't believe how many people screw this up.)
Warning: After five beers, swimming skills deteriorate. Remember, drowning is both embarrassing and a bummer.