Cream cheese and Lox

You can whip your own cream cheese--use the low fat variety (Neufchâtel), just let it sit at room temperature until soft, and beat it with your mixer paddle. Trader Joe's sells lox 'ends and pieces' which are a good value and mince up just as easily as full size lox strips...I actually buy that even when I want full slices because the sizes are good. Add some minced chives or scallions and serve with cucumber slices. Capers if you're feeling flush.

It could be good to include some fresh herbs?
http://allrecipes.com/recipe/15361/smoked-salmon-spread/

I've also had a cream cheese spread with candied salmon and it's delicious!

Minced smoked salmon folded it into well softened or whipped cream cheese, is very easy to spread - I don't think you need to add anything. You could serve a classic cream cheese with minced scallion as well. (Btw, my local deli sells the odd shaped smoked salmon fragments at a much lower price - perfect for this purpose, makes upping the ratio a little easier. Yours might as well.)

Thanks for the answers but I need to keep this simple for both cost and logistics reasons. This is not a situation I feel any need to be impressive. Of course I planned to also provide plain cream cheese. I was thinking more along the lines of tips for making a spreadable lox spread( add milk?). I see now why the idea of serving the lox separate keeps being discussed. What I meant when I said "just cream cheese and lox" was as the ingredients to use in the spread, not to serve them separately. Thanks for chiming in.

Depending on your crowd, keeping the old and cream cheese separate could get very speedy. However, the nice thing about keeping it separate is that people who don't care for old can just put plain cream cheese on their bagel. Whipped cream cheese is much easier to spread, but blocks are the real thing and I prefer the texture. Again, your choice depending on the crowd. You may want to also include some thinly sliced cucumbers and ice water soaked paper thin red onions..and a small bowl of Altoids. :)

Agree with Lisanne that the salmon spread was a way to stretch the (expensive) lox.
But if you like the idea of a spread, or want to give people a choice, there are two I would recommend, one British (smoked mackerel, from Felicity Cloake, a columnist at the Guardian) and one Yiddish (smoked whitefish spread, a classic deli offering).
guardian.co.uk/lifeandstyle/wordofmouth/2012/may/24/how-perfect-smoked-mackerel-pate
epicurious.com/recipes/food/reviews/mark-siegels-whitefish-salad-40022

It depends on the size size of your group & resources. Lox spread is delicious but I think was originally devised as a money saving way to stretch pricey smoked salmon. If you can, I would stick with straight smoked salmon as a more luxe option...depending :)
Otherwise, just soften cream cheese blocks and mince some smoked salmon, cream it in.

If you want to go all out, you could make beet-cured salmon (https://food52.com/recipes/34105-beet-cured-salmon) and smoked carrot "lox" for any vegetarians (https://food52.com/recipes/51602-smoked-carrot-lox)