I have to go out of town a lot, so I know how you feel here. First of all, yes I suppose you could just run the fluval. I'm assuming of course that this is fully separate from the sump. You may need some additional current in your tank to make up for the lost pressure the sump return pump provided. BTW, what's in the sump? If there's going to be standing water in the sump for the week, you should REALLY consider putting a powerhead down there for the time being to keep things mixed and moving. If you have bioballs, make sure they don't dry out - if they do, it may kill a lot of bacteria, and create a potential deathtrap when you get back from all the ammonia.
Instead of shutting the sump down, is there a kid in the neighborhood that you trust to come in and do basic maintenance while you're gone? You sound like you know how much your sump is evaporating; you could pre-measure the daily topoff water into gallon jugs and set them out for the kid to add. While he's there he could swipe the glass, etc.... most kids would be happy to do this for a little $$, and it may even inspire someone to try the hobby themselves.