The Rainbow Bridge is a land border crossing between Canada and the US. There are no facilities to cross it without departing one country and entering the other, so as a result you will need whatever legal status is required to enter the other country - which obviously depends on your citizenship.
If you are a Canadian or US citizen then you do not normally need a Visa, but you will require a passport or a passport card. There are exceptions to this if you have a criminal record, etc, but these are the same for any Canadian citizen entering the US or vice versa.
If you are a member of a Visa Waiver Program (VWP) for entering the US then you do not normally need a visa (again, with the usual exceptions around criminal records, etc). As you are entering by land you do not need an approved ETSA as you would if entering by air, however you will need to fill in an I94W form.
If you are from a country that requires a visa to enter the US, then you will require a visa. Technically if you do not have a visa then the US would most likely "deport" you to the country you arrived from (ie, back over the bridge to Canada) - do not think of this as an easy way to get around the requirement if you are just expecting to see the bridge. Doing this will give you a permanent mark against your US immigration record, and make it harder to enter the US in the future!
The main thing you need to keep in mind is that you will be departing from and then re-entering Canada, and thus if you require a visa for entering Canada you should make sure that it will allow you to re-enter. eg, if you are on a single entry visa for Canada, you will technically not be allowed re-enter after visiting the USA.