public void saveaction()
    {
        system.debug('saveaction inside===>'+reg1);
        try{
            upsert reg1;
            Hotel__r.Registration__c=reg1.01I5g000001paEN;
            AirTravel__r.Registration__c=reg1.01I5g000001paGd;
            if(reg1.Id !=null){
                ApexPages.addMessage(new ApexPages.Message(ApexPages.SEVERITY.INFO,'Record saved successfully'));
                reg1 = new Registration1__c();
           } else
            {
                ApexPages.addMessage(new ApexPages.Message(ApexPages.SEVERITY.ERROR,'Record not saved')); 
            }
        }
        Catch(Exception e)  
        {
            System.debug(e.getMessage());
        }
    }



<apex:page controller="Registrationcontroller" id="theRepeat" sidebar="false" showHeader="true">
    <apex:form >
        <apex:pageBlock >
            <apex:messages />
            <apex:pageBlockSection columns="1" >
                <apex:inputField label="FirstName" value="{!reg1.FirstName__c}"/>
                <apex:inputField label="LastName" value="{!reg1.LastName__c}"/>
                <apex:inputField label="Email" value="{!reg1.Email__c}"/>
            </apex:pageBlockSection>
            <apex:pageBlockButtons >
                <apex:commandButton value="save" action="{!saveaction}" rerender="pg1,theTable" />
            </apex:pageBlockButtons> 
        </apex:pageBlock>
        <apex:pageBlock >
            <apex:dataTable value="{!registerlist}" var="regvalue" id="theTable" width="100%">
                <apex:facet name="caption">Registration List</apex:facet>
                <apex:column headerValue="Reg ID"><apex:outputText value="{!regvalue.Name}" /> </apex:column>
                <apex:column headerValue="FirstName"><apex:outputText value="{!regvalue.FirstName__c}" /> </apex:column>
                <apex:column headerValue="LastName"><apex:outputText value="{!regvalue.LastName__c}" /> </apex:column>
                <apex:column headerValue="Email"><apex:outputText value="{!regvalue.Email__c}" /> </apex:column>
                
                    <apex:repeat value="{!regvalue.Hotels__r}" var="c">
                        <apex:column value="{!c.Name}"/>
                    </apex:repeat>
                    <apex:repeat value="{!regvalue.AirTravels__r}" var="d">
                        <apex:column value="{!d.Name}"/>
                    </apex:repeat>
               
                <apex:column >
                    <apex:commandLink value="Edit" action="{!editfunction}" rerender="pg1">
                        <apex:param name="cid" value="{!regvalue.id}" assignto="{!editid}"/>
                    </apex:commandLink> || 
                    <apex:commandLink value="Delete" action="{!deletefunction}" rerender="pg1,theTable">
                        <apex:param name="cid" value="{!regvalue.id}" assignto="{!editid}"/>
                    </apex:commandLink> 
                </apex:column>    
            </apex:dataTable>
        </apex:pageBlock>
    </apex:form>         
</apex:page>




Custom Object - AirTravel, Fields - Arrival and Departure Datatype- Picklist. Custom Object - Hotel, Fields - Check In Date and Check Out Date Datatype- Picklist. These two custom objects lookup with Registraton1__c. How to give Hotels and Airtravel lookupvalues in reagistration1__c