Is it easy for someone to show up and just get a spot without a reservation on a Tuesday?

I would not advise that. You may find a spot that is open but it may be reserved via the site and that reservation is in route. They may arrive late and if someone is there, they must leave. Just checked website and all spots are full today... https://www.venturaparks.org/payment/

It's difficult to say... I thought it might be easy to go on a weekday - mid-week, not summer, but there were very few spots available. For same-day reservations, you can go to their website and choose Mobile Pay https://www.venturaparks.org/payment (without choosing a date because they don't allow reservations for same-day) and then after you begin that page, you can see if any spots are open to reserve and pay immediately. Otherwise if you just try to take an empty spot, a Park Ranger will be by sooner rather than later to let you know that they can fine you $75 on the spot! That's how we found the backdoor way to check on openings same-day. Hope this helps! It's so beautiful there :)