Use input field value from standard controller extension property

I am trying to get an input field value to reference a standard controller extension property.

Here is my controller extension:

public class LicenseActivationController {
    private final Contact contact;

    public LicenseActivationController(ApexPages.StandardController controller) {
        contact = (Contact)controller.getRecord();
    }

    public String activationID { get; set; }

    public String computerID { get; set; }
}

Here is my VisualForce page:

<apex:page standardController="Contact" extensions="LicenseActivationController">
    <apex:form>
        <apex:pageBlock title="License Activation">
            <apex:pageBlockSection columns="1">
                <apex:inputField value="{!activationID}" />
                <apex:inputField value="{!computerID}" />
            </apex:pageBlockSection>
            <apex:pageBlockButtons >
                <apex:commandButton value="Activate" />
            </apex:pageBlockButtons>
        </apex:pageBlock>
    </apex:form>
</apex:page>

As you can see, I am trying to use activationID and computerID properties in the corresponding inputField, but the error I get is:

Error: Could not resolve the entity from value binding '{!activationID}'. can only be used with SObjects, or objects that are Visualforce field component resolvable.

How can I achieve extending the Contact controller and using properties inside the extension class like I am trying to do here?