You can use conduit above ground provided that it is rated for how you want to use it (i.e., if it's going to be outside above ground it has to be rated for sun exposure. Note you need to use conductors rated for wet locations (underground is always considered wet and I suspect your overground runs are sufficiently exposed to be considered so as well).

You have a couple of misconceptions here. Unless you have some local code that says otherwise there's no requirement for "a full sized ground" just one that's at least 12gauge and appropriately sized for the circuit. What is required is that it be insulated. However, if we're talking about a single family home, there's an exception that allows you to use a bare ground that's enclosed inside the sheath of a cable (e.g., romex) for the part of the circuit that's inside the building.

The instructions for the hot tub said it has to be a full sized ground. If I can get away without it I might just run 6/3 through the attic to the spa panel on the side of the house and then run separate conductors from there through underground conduit to the spa.

If the instructions for the spa say it's required (rather than recommended) then you have to do it.

Does your spa really require a NEUTRAL? If not, you can use the white wire in the 6-3 on the ground if you remark the exposed sections of the wire you're using as ground green.

tonys-
Regarding your original post, I'm a little confused. Conduit does not take up much space. Why can't you just put conduit in the ground next to the pad, or even run it under the edge of the pad by just digging down and in a little?

I would run it right next to the pad but there is a stone retaining wall all the way up against the cement. That wall takes up every bit of space between the pad and the property line. I'm scared if I dig under it the foundation for the wall would be compromised.

I know its been a while but I've been busy and haven't had a chance to mess with this. I was thinking about it a little more and have another question. Isn't rhe full size ground only required from the spa to the spa panel on the house or all the way back to the main panel? Seems to me that the wiring wouldn't make a difference between the spa panel and the main panel. It would be the same as me wiring in the spa to a sub panel correct? I know that they don't use a full 6AWG ground. It would be nice to be able to run 6/3 romex through the attic to the spa panel on the side of the house and then conduit with individual conductors to the spa.

Use rigid metal or intermediate conduit and you only need 6 inches of cover