The Hydra (a harrier jump jet, also in gta:sa) is quite a bit of fun. My brother purchased one today for $3 million.
You can get it to hover and then land a helicopter on it.
And I finally got myself the cheapest high-end house, for $200,000. So I've now hosted my own heists. But I only did the first 2 player one with my brother. We're possibly going to do the next prison break one tomorrow if we can get lucky and get two competent players to join us.
Also, a tip to all new PC players who haven't played online on a console before, when you need easy money play Survival games. The Processed map in particular has some perfect covers for fighting from. You can earn $30,000 for completing all 10 waves each time, also about $4000-5000 in dropped cash if you're quick. Furthermore the Survival provides you with weapons and ammo as you go along, which gets increasingly more powerful as the waves progress. This can save you good amounts of money on ammo, especially explosives. And you'll get high level weapons you otherwise can't get at low levels such as carbines, snipers, rocket launchers and miniguns. These will stay with you outside of the Survival and will persist for your current online session or until you run out of ammo. So they're very good if you're low level and need better weapons than your pistol for missions or fighting other people in freeroam.