I have a yellow and green clown goby and they are really cute. They are active about half the time and perch the other half. lol
Will this be a reef tank with corals or just fish? Do you intend to go with corals at any time? How about inverts like snails and hermits? Theres a bunch of fish you can choose from but you have to consider those questions when picking your fish.
I like lawnmower blenny's, a super ton of personality, havent met one yet that wasnt a character. I also like citron gobies another not usual fish but good for a 20g tank.