If you would like to spend at least half a day 逛台場, personal suggestion is just buy single ticket to and from 台場 (as I remember, It costs 190 Yen for single trip between 新橋 and 台場 by Toei subway line, so 380 Yen get you there and back) and walk in that no-so-big area.
富士電視台, 維納斯堡, 海濱公園, 彩虹大橋, Aqua city 這幾個景點 are all within walking distance. Don't worry. 原文載於: 背包客棧自助旅行論壇 http://www.backpackers.com.tw/forum/showthread.php?p=2246952
Using 一日券 saves you some strentgh, but you need to get in and out of subway station (with long escalator) and wait. For bus, you need to get in line and wait. These make a person tired, as well.
Decide for youself. You would have a great time in 台場 either way.