My contractor installed particleboard kitchen cabinets that have a oak doors. These look great from the outside but the particleboard seems to scream "cheap". Although I can't do anything about how they look inside, I was wondering what the best way to protect them from water damage? Do I paint the insides? Place contact paper? Are their any other options?

Best long term solution is to specify in the lease that there is to be no water usage.....ever. If for some reason that is not agreeable with your tenants, then you are stuck relying on shelf liner.

A few other options are: 1) let the process take its course and know that you will have issues to deal with in a few years, although these are relatively durable products assuming proper use and damage control 2) try to water proof them with a fast drying brush on lacquer product or an aerosol product (several thin coats)....never done it but don't see why it couldn't work or 3) after step 1 is complete and it has been a few years, you can sand down the bubbled up cabinet bottoms where wet plates and cups were set and put new 1/4' hardi-board???? that brown stuff that looks like peg board but without the peg holes in it to create a new bottom and paint it white to match. If you barely glue it down in a few spots, or attach with thin double sided tape, you can replace that every few years as needed. Alternatively, you could find other veneer products etc...up to you for cost.

They may not last forever but if particle board was THAT terrible, IKEA would not be around.

I did find this idea when I searched online: Use peel and stick vinyl floor tiles. Here is the link: http://www.todayshomeowner.com/video/how-to-protect-a-sink-cabinet-from-water-damage/

I'm not sure if Iike the end result or not.