Paris trip - booking excursions

[MikeyT]

Hi all,

Is it better to book excursions - boat trip, shows etc when we're in Paris or over t'internet before we go?

Are prices cheaper if booking using the latter method (over the net and early - we go in three weeks) or does it get more expensive if we try and book when we're there through the hotel or travel services?

Thanks

[MikeyT]

We're booked on Eurostar but it was a raffle prize so it's already booked for us ... as is the notel (nice)

Just wondering whether we should book though the net now for anything or wait until we get there ...