I think that a lava lamp is a pretty decent science project, as long as you go into the science behind why the blobs of wax move up and down. This type of thing is called "convective heating / cooling" and is understood pretty well by scientists (although modeling it using equations or computers is sometimes difficult). If you have a decent blurb on how/why it occurs, then it is a decent project to conduct!

Also, if you're building a lava lamp yourself, I advise you to be careful! Some of the heating is pretty intense and some materials could potentially catch on fire if they get too close to the heating coils. Although if you don't build your own lava-lamp, it's not nearly as interesting or fun =P

It sounds like it would be a lot of fun to make a lava lamp!But I don't think it's a good science fair project on its own. A good science project starts with a question. Here are some questions that you might try to answer by experimenting with your lava lamp.

How hot does the lava lamp need to be for the blobs to move up and down? How big are the blobs, and does this depends on the size of the lamp or the temperature of the lamp? Can you change the size of the blobs by using a different kind of wax? What if you change the liquid the blobs are floating in? What happens if you use a different light bulb?

Whenever we do science, we try to answer questions. Try to come up with a few questions of your own!

Good luck!