I "searched" this same topic ended up here. I have the same think going on with my 1990 Camry 3S-FE with about 80k miles on the engine. It is odd that my (current) daily driven 1988 Camry 3S-FE with 180k miles on it does not have this problem.
I had this problem on my 1990 Tercel with the 3E-E. I didn't take the head off the (engine) block. The Tercel head was not a hemi like the Camry. I don't know if this same technique would work for the 3S-FE in the Camry.
Buy a new set of valve stem seals from an auto parts store. When I did this they came with a little plastic drinking straw like tool that was slid over the valve stem. Once the old seal was removed the tool was slid on the valve steam and the new seal with a little engine oil on it was slid over the valve steam seal and into place. A small socket the same diameter of the seal would fully seal it when pushed on it from the top.
The trick to getting the valve springs and keepers off without the valve falling down into the cylinder is what has to be figured out. Two methods I've heard of, one I've used. The first is using compressed air. Make a tool out of an old spark plug that an air line from a compressor can be attached to or use a leak down tester with the correct spark plug thread. Thread this into the head in place of the spark plug and fill the cylinder with air. This will push up on the valve and hold it in place while the keepers are off. I never trusted this with a cold old engine. I wasn't sure how good it would seal and hold the valve up. One drops and you are SOL and taking the head off new. The other method was to use small rope. I've used about 4' of 1/4" nylon, anything that will fit through the spark plug hole. Remove the spark plug from cylinders 1 and 4, then later 2 and 3. For ease start with cylinder 2 and 3, set the crank position to TDC. Remove the spark plugs from cylinder 2 and 3. Stuff as much rope through the spark plug hole (about 3 feet worth) with a needle nose pliers. Then turn the crank clockwise to as close to 180° as possible. There will be resistance, you can only go so far. This is the idea, the rope is being pushed against the top of the cylinder and the valves. Now when the keepers are removed the valves will not fall into the cylinder. After all this you do the same thing with cylinders 1 and 4, but opposite crank positions. It is a time consuming process stuffing the rope in.
I have never done this on a 3S-FE but have done it twice on a 3E-E and a 3E engine. Worked good both times.
Not sure if I did something wrong with the new valve steams they didn't seem to seal a whole lot better, mostly at first. I'm sure it will take some time for them to seal in?