Making a tuna salad sandwich with diced apples and celery. What would be a good herb or spice to add to add some oomf.

I'd use a curry blend! That's one of my favorite, all-time combinations. I'd add a few chopped toasted almonds right at the end, too. ;o)

Chopped sweet dill pickle - like the green relish kind.

I have to vote for dill, but now that I think of it, toasted and ground cumin would be delicious here.

I love tuna with apples! I'd add finely chopped arugula, fennel, celery or celery leaves.
http://www.food52.com/recipes/6984_crunchy_tuna_salad_with_apples

I happen to love tuna with finely chopped apple, craisins, and dill. You can also add pickled red onion or very finely chopped red onion. I season with a mayo, olive oil, and lemon juice concotion. Salt and pepper.

fennel would be delicious.

I like tuna salad with apple - assuming we're talking tuna w/mayo. I'd go for the dill. Also maybe a little horseradish - apple and horseradish are friends and I'm thinking salmon goes well with a mayo-based dill/horseradish sauce, so why not tuna? (A sprinkling of snipped chives wouldn't hurt either.) Maybe on some good pumpernickel.

Tarragon Was my first thought as well. The tuna apple match up is tricky.

Not that I'm disagreeing but I'm thinking Grains of Paradise

I vote for the tarragon! But then I always come out in favor of the Mediterranean over Scandinavia!

Dill (coarsely chopped, longer stems removed), especially if you're using a simple mayo or mayo/mustard dressing. To your mayo mix, you could possibly add some heat: a dash of tabasco, sriracha or a small dash of cayenne pepper.

I think tarragon would be fabulous!