I am looking at the expansions and I don't think I am quite ready to add another board.

Mainly what I am looking for in an expansion is additional location cards, item cards, new characters, etc. I like the game as is, but just want a little more variety (especially with locations), but not necessarily looking for new mechanics.

Any thoughts on which card expansion is the best?

Your answer may actually be a big board. No small box expansion has additional Investigators or AOs. And Dunwich (a big box) has the most Location cards, giving you 7 more per location (not including the new locations). You can ignore the board, and some of the Mythos cards that deal with the board. Other than that, no new elements arise aside from Injury/Madness cards (but those are VERY beneficial to you). Plus, you get 8 new investigators and 4 new AOs.

Curse of the Dark Pharaoh is a small box that has 5 new location cards, if that's what you're looking for, and doesn't add much else besides theme and some neat things. Every other expansion, big or small, has 4 new location cards for the base game. If you can't find Dunwich, or don't want to spend that much, I'd suggest Dark Pharaoh if you haven't quite conquered the game just yet. If you find the game too easy, go for King in Yellow. Either way, most people suggest getting Lurker at the Threshold as your second or third expansion. If you don't mind getting a big box, get Dunwich. Then I'd go for Innsmouth (after a few small expansions), so you can get TONS of new Investigators and AOs!

I think Ecno said it best. Get a big-box and just don't use the board for a while. 99% of the components don't require the board. In fact, you can place the board underneath the Arkham board and leave only the new Other Worlds exposed, so that you can use Dunwich's gate tokens and gate cards.

I think Lurker might be the best small expansion, at least as far as components go, but Dark Pharaoh has the most encounter cards of all the small boxes (5 instead of 4—a small margin), and Dark Pharaoh definitely has the best theme of any expansion. But its non-encounter components aren't the best.

I got Lurker recently (wasn't ready to shell out for a big box quite yet, though considering doing so soon), and am really liking it so far. It has a lot of good stuff that can work with your game whether you're using the Herald mechanic or not. I can't speak for the other card expansions, though, being rather new myself.

But yeah, it's important to note that you never have to use all the components in an expansion. If you aren't ready for a new board but you want a ton of new cards, Investigators, and AOs, just get a big box and ignore the board for a while. The expansions are delightfully customizable.

Also, if you're just looking for some new characters or AOs, you may want to check out the Fan Creations section of the board. There are some very interesting concepts available there (and not just saying that because I've made some).