I found triangle shaped markers made by Crayola that stay on the table!  They are for 18 mos and up and are washable too.  My kids love them and they wash off very easily with just plain water.  They are a bit wide though so some kids might have a hard time getting used to gripping it but once my kids got used to it, it was not an issue.  Plus my kids liked not having to hear me nag them about picking them off the floor!