Race some cars in Mexico! My personal favorite is the E46 M3. I just love beating those, not sure why but it always makes my day.

You could try making runs out at the local track or dragstrip. I ran some 1/4 mile runs for the first time at a strip last weekend and all I can say is when you're dead even with that car next to you and then start pulling on them - man, what a rush! I have an entire new appreciation for horsepower and torque!

This is what I hate about NA cars...they get boring unless you spend huge$$$$$$$ and make huge changes. Gte tired of the E55, chip it and BOOM

Sell the C55 and buy a CTS-V. While it doesn't have the pedigree of the AMG or an M, or other Euro Super Saloons, it will quickly put you back in love with whatever it was that convinced you to buy a Super Saloon in the first place. Keep the SLK55,... maybe even do some suspension mods on it and let that be your ride when you seek adventure. I believe you'd be surprised at how well the CTS-V will meet your needs to handle an imperfect surface, the ease it haul things and people, and no shoe horn needed. Besides,... it's faster than anything mentioned in here thus far.