What sounds like fun to you? What's your budget?
There are many 8/16 bit controllers that would likely feel very nostalgic for a bit. (since they really are basically the same as what you used to play with). You'd be surprised how many professional embedded systems still use 8051s.
An Arduino would give you a lot of interesting add-on choices, but to me it really seems geared more for hobbyists who have to program to accomplish a task... not to people who like to get down and dirty with C and ASM. If you want a lot of embedded control options, this is good though. Be sure to check out SparkFun.
Propeller looks fun... although it never seems to fit anything I'm doing. I restarted my electronics interest maybe ten years ago with Parallax's Basic Stamp II (VERY quickly moved to their SX stuff (PIC-like system, now Ubicom))
ARM processors vary widely. On the low end, they're powerful but the tools usually cost too much. On the medium and high end, they can run a full Linux system and do amazing things. (Just about all phones use them now) I'm moving into the medium end ARM range myself (lowest end capable of Linux, really) My definition of medium fits the chips that are simple enough to design a 4-6 layer PCB for. The big ones can require 8+ layers.
If I was to take a guess, I'd say that after a few months getting your sea legs back you might find FPGAs really interesting. You can get nice FPGA demo boards for $100-$200. The vendor tools are free (I'd recommend Altera followed closely by Xilinx) You can have a lot of fun designing your own logic, including small microprocessors. There are a LOT of 'soft' processors for FPGAs, and complete toolchains for developing and debugging them. I'd guess that it best fits the feeling of the late 70s/early 80s (not that I'd know) in that you can design so much of a system yourself from scratch. You can then go find just about any sensor or device whatever you want, and write custom glue logic to talk to it (no matter what bizarre interface they're using) There's also OpenCores, where you can grab functional modules that people have written if you don't want to write it yourself or don't know how. If you're interested, comment and I'll look for a decent starter board for you.
Unfortunately, you can't go TOO far with FPGAs (unless you really like playing with logic alone and not interfacing with things) without also designing PCBs yourself. I think it's a lot easier than it sounds... datasheets these days pretty much tell you how to do everything. However, it's certainly a whole lot more to manage.