The RATP is a very developed net. You can you get a package of 10 tickets +, including the subway, RER, bus, tramway and funicular of Montmartre (except intinararies by TER, Orlybus and Roissybus). The ticket at unit costs 1,70 € and the package 13,70 €.
You can also buy the unlimited pass "Paris visits". It covers zones 1 - 3 or 1 - 5 of RATP and SNCF nets (can be used for Orlyval). And it's valid from 1 to 5 consecutive days, according to the time of your stay. It also allows to obtain reductions on the visits among 13 big touristic sites.
Otherwise, you can also use the vélib' or the autolib' (rental of electric cars).
Avoid taking the taxi because the traffic is often very dense.