The sea. The pacific already has a billions of tonnes of natural radioactives in it, so diluting the cooling water into the ocean slowly would be safe.

Uhm. done correctly, anyway. One needs to avoid local concentrations at any point.

It might be possible to form a salt that either would precipitate or that could be evaporated from solution. Then they could build a chemical plant of a scale sufficient to chemically react 99% of the cesium and store the dry chemical in suitable containers. Cesium is nasty stuff. From wiki:

Chemical properties

Caesium metal is highly reactive and very pyrophoric. In addition to igniting spontaneously in air, it reacts explosively with water even at low temperatures, more so than other members of the first group of the periodic table.[7] The reaction with solid water occurs at temperatures as low as −116 °C (−177 °F).[11] Because of its high reactivity, the metal is classified as a hazardous material. It is stored and shipped in dry saturated hydrocarbons such as mineral oil. Similarly, it must be handled under inert gas such as argon. However, a caesium-water explosion is often less powerful than a sodium-water explosion with a similar amount of sodium. This is because caesium explodes instantly upon contact with water, leaving little time for hydrogen to accumulate.[17] Caesium can be stored in vacuum-sealed borosilicate glass ampoules. In quantities of more than about 100 grams (3.5 oz), caesium is shipped in hermetically sealed, stainless steel containers.[7]

....

Compounds

The vast majority of caesium compounds contain the element as the cation Cs+, which binds ionically to a wide variety of anions. One noteworthy exception is provided by the caeside anion (Cs−).[21] Other exceptions include the several suboxides (see section on oxides below).

Returning to more normal compounds, salts of Cs+ are almost invariably colorless unless the anion itself is colored. Many of the simple salts are hygroscopic, but less so than the corresponding salts of the lighter alkali metals. The phosphate,[22] acetate, carbonate, halides, oxide, nitrate, and sulfate salts are water-soluble. Double salts are often less soluble, and the low solubility of caesium aluminium sulfate is exploited in the purification of Cs from its ores.

So storing radioactive cesium in its metallic form seems unfeasible at scale, but perhaps as caesium aluminium sulfate it could be stored as a solid. But there would still be the problem of keeping it cool.

A special purpose chemical plant capable of reacting the cesium in the wastewater at three times the scale it is currently being generated and storing it in a secure area, sufficiently seismically stable as to allow for a survivable facility design and above the highest level tsunami have ever reached, could well be technically feasible. But don't expect TEPCO to do it.

Such a facility may cost $50 million or more to build, but it would deal with an ongoing severe problem. The Japanese government should form a separate organization that would be funded directly by the government and charged with doing what is needed in a reasonably cost effective method but not constrained by 'how it is going to be paid for'. It is, after all, the viability of a large area just north of Tokyo and fishing industry for the Pacific shore that is at stake. And dealing with this issue aggressively would be a boon to the economy just now.  

"It is not necessary to have hope in order to persevere."