You could have one or two small, low bioload fish, and then focus on cool inverts (like scarlet reef hermits, small colorful shrimp, small feather duster, a couple small snails.) This is what I have in my 12 gallon nano, and it is a very active, colorful tank. Add in some zoa and mushroom frags, and a bit of decorative macro and/or a small ball of chaetomorpha, and you could have a nice little tank. It is very easy to overload a 10 gallon tank with fish.
The smallest size for a decent marine fish tank, IMO is a 25 or better yet, a 29.
FYI, in my 12 gallon nano:
Several really nice looking pieces of cured live rock
High fin red banded shrimp goby
http://www.liveaquaria.com/product/prod_display.cfm?c=15+2124+181&pcatid=181
Small tiger pistol shrimp (may have to move and replace with smaller pistol as it grows)
http://www.liveaquaria.com/product/prod_display.cfm?c=497+525+1148&pcatid=1148
Several small saltwater ghost shrimp (feeders for my scorpions) - I put a few in this tank, they are very active and engaging
Two scarlet reef hermits
Assorted snails, including small Nassarius, Nerites
Dwarf feather duster - pink and white, a centerpiece in the tank
Assorted zoa and mushroom frags
Chaetomorpha
Some pretty red macroalgae - two species (came up on the rock)