Dig up fossils around your town (stars on the ground) and have Blathers check them. Be sure not to donate it, even when he asks if he can keep it. Afterwards, sell them to Nook. Most fossils are worth up to 2,000 Bells.
Dig up a load of fossils before a Flea Market day. and keep them. Then, take all the furniture out of your house and fill it with the fossils. Do this on the day before or else the animals pop in after you close an inventory screen, try to buy things, and they do not come back. Sometimes you can sell them in the region of 10,000 Bells per fossil.
There is a wolf named Wolfgang that sometimes appears during the flea market day. If you set up a Full Body Dinosaur Set, you can set the price up to 200,000 Bells. Also, if it is early (between 6:00 a.m. and 8:00 a.m.), he will be so tired that he might pay up to 450,000 Bells.