If the old Mac still works turning it into a fish tank or op-art is not a good idea.
If you no longer want it sell it on ebay... it WILL sell....
some uses have been mentioned and I have used older Macs for these tasks at some time or another.
Digital answering mailbox unit. you can even have fax back features , etc in case you have a small business. Our phone system runs on a 7200 an se 30 before that.
A small server like a mail server or simple web server.
If you have children in the family (young children under 6) you could paint it a nice bright color and put some simple learning software, etc on it to get them aclimated to computers, if it gets screwed up no worries
macbsd or freebsd is certainly another idea
with a little work you can create a simple dedicated mail machine for sending and receiving mail. without having to know how to use a computer much. this would be great for a grandparent that hates computers. make it simple enough to just turn on and the app starts and gets the mail for them. applescript is great for that.
simple old print server
these are only a few ideas, I am a mac tech in NYC and have seen Macs of all ages used for all sorts of things... old macs are still used in old news papper and print shops.
cheers