If you really plan to be in Downtown Disney all day, don't park in the Simba lot. Even with Validation, it will cost you plenty. It would definitely be cheaper to park at Mickey and Friends and take the tram in. I believe Mickey and Friends is open by 7 (though sometimes they start letting cars in earlier than that).
Now, if you're only going to be at DTD for 6ish hours, then park in Simba, eat at a table service restaurant and get validated to get 5 of those hours for free.