Now Thank We All Our God - at the cyberhymnal here, but IMO it's a bit formal, almost like a Bach chorale. I knew a fella who used it for the recessional at his wedding!

I like Alleluia (even though it doesn't mean thanks) for a capella singing. The alto part is beautiful. The only words in the first verse are alleluia, so the congregation could sing along too. It's in the 1975 Baptist Hymnal, #422, but I only found one midi on the net, a rather lame one at that. this page, scroll down to alleluia - it's a bit hard to hear with my inferior sound card - just remember that the harmonies are beautiful and the melody extremely easy.

You could sing both of them - ending with Alleluia. ;-)