The Classic Birkenstock:
"Made in German, tradition since 1774" -- we say, you can't go wrong with these. Check out their website: http://www.birkenstockusa.com/men for more info. To pricey for you? Many stores sell cheaper, knock off Birkenstocks, also acceptable.
Yes, these are acceptable, but not for everyone. Check out some cool styles below, available at ae.com.
The Cross Over:
An acceptable alternative to the flip flip, but possibly more comfortable for some; check out a great pair of Reef cross-overs, available at http://www.reef.com. A great leather pair of cross-overs would also be acceptable.
A Quality Pair of Leather Sandals:
You can't go wrong with a sturdy, comfortable pair of well made leather sandals. Check out these Fossil pairs, available at: http://www.shoes.com.