Generally speaking, there is no need to have terms of service for the data that you provide. At best they accomplish nothing, at worst they prevent the data from being reused (removing the “open” from “open data”).
Terms of service may be used to prohibit people from engaging in behavior that would compromise the integrity of government servers (e.g., rate-limiting API queries), but they must not prohibit people from doing anything with data that they see fit. Requirements that the agency be credited, that the data not be altered, etc. are counter to open data norms, and will be an obstacle to realizing the benefits of publishing open data.
That said, it’s entirely possible that somebody has told you that you have to include terms of service. And now you need to figure out how to comply with that without doing any damage.
If you must have terms of service, emulate the one used by the U.S. Food and Drug Administration for their open data repository. In fact, you don’t have to “emulate” it—you can just take it, since it's an uncopyrightable U.S. government work. And it’s been thoroughly vetted by the FDA, so you know it’s legally solid. Just replace every mention of the FDA with the name of your own organization, change the references to federal law to state law, etc., and you’re good to go.
For more information, read the “Additional Notes” section of Open Government Data’s “Best-Practices Language for Making Data ‘License-Free’.”