I think Reaktor would be a great fun especially with the great free and commercial ensembles available. I don't have the full version but I do have Razor and Molekular which can be the most what I want from Reaktor
I don't know if Tone2 deal is still available (2for1), but I jumped in and got Nemesis and RayBlaster. I love them. They are excellent synths and with a low cpu usage. They are so fun to program and to play with. For about one month and I'm still having a great fun and it is actually increasing with time!
Other big fun is Reason for me! Can't beat the great inspiration and the unlimited creativity. Also, there are many advanced users and watching their techniques really opens up so much creativity and fun even if they do some other genres that I don't prefer.
Lastly is the more capable you know and use your tools, the more fun you get. You might not need anything else, but we always want something else
edit: for the sleepy typos and grammar errors!

