Now that I have my flights and hotels set up. I wanted to know if it was best for me to purchase my sightseeing tours ahead of time and if so what is the best sights online? I will be in London,Paris,Brussels,and Amsterdam and I wanted to do all the touristy stuff so I would like tickets for the Eiffel Tower, London Eye, Hop on Hop off Tours ...

I don't see any important advantage in purchasing tickets for any of the things that you mentioned before your arrival. Moreover, you might find that you can obtain special deals for some of them (but perhaps not). You may also find that you don't want so much in the way of tours when you find out how easy it is to do things on your own. That's my two cents worth anyway.

I'm with Flyboy. I was tempted at first to say pre-purchase your ticket for the London Eye to avoid long lines, but what if you get there and you suddenly have a change of plans due to weather, strikes, or something else beyond anyone's control? These same factors could apply to the Eiffel Tower. As for the hop-on, hop-off bus, it takes about 30 seconds to buy that ticket, so there's no reason to pre-book that either.

You don't have to pre-schedule everything. Half the fun is getting there, then deciding what you want to do with that particular day. Play it by ear, and have fun!!

Absolutely do not buy these things in advance - it ties you in to way to much.

First of all - you don;t know how you're going to feel any specific day - busy or relaxed.

Second, who knows what the weather will be each day - indoors or outdoors?

Third - it leaves you no room for serendipity - which is one of the joys of travel.

I think there are a couple of exceptions where booking well in advance of your trip is a considerable advantage, but I'm not thinking of any in the cities you mentioned.

I'd book on line in advance the Alhambra in Granada, Spain. (We booked months ahead of time and the morning tickets were already sold out) and (I think) the Ufizzi gallery in Florence, where we were in line over an hour.