Option 1 is to fit the processor from the now-deceased Aether v1. That's I believe a Pentium M 715 1.5GHz Dothan processor, which is the successor to the Banias core. It's clocked slightly slower than the Banias, but Dothan-based processors are supposed to give a 5-10% performance increase which works out about equal for those clock speeds. The net effect of that upgrade will probably be no change in performance and a slight improvement in battery life.
Option 2 is to go all-out and buy the fastest processor that'll fit (or rather, the fastest processor that can be got for a reasonable price). That would probably be the Pentium M 755 2.0GHz - I could go for the 765 2.1GHz, but that costs twice as much on eBay. Upgrading this far means I have to start thinking about power and cooling as it's not like a desktop where I can just stick a bigger heatsink/fan on the processor, but rather I'm stuck with the cooling system of the laptop which will be designed around a specific thermal output. I think I can get away with this, as by moving from the Banias to the Dothan core the rated TDP of the processor actually decreases by a few watts. Not sure it'd help the battery life though.
Verdict: option 1 for now.
Another bonkers upgrade is adding more memory. It always used to be the case that the most cost-effective computer upgrade was to double the amount of RAM - these days, I think replacing the boot disk with a SSD is possibly more effective, but I've already done that. As it stands Aether already has had one memory upgrade (The Gnu ordered it with the laptop) and contains 1.5GB of RAM. I think DDR SO-DIMMs top out at 1GB so the best I could manage is a total of 2GB. That's probably not worth doing unless I happen to have a spare one somewhere - and I don't think I've got any spares large enough to be useful.
Finally, a replacement battery wouldn't go amiss. New batteries for a ThinkPad R50p are unobtainium these days but there's a handful of outfits that do refurbished batteries and various eBay sellers hawking second-hand ones of varying capacity. There's also the option of replacing the CD/DVD drive with a second battery, assuming such can be found - even when the R50 series was new those were rather rare.
Verdict: get a replacement main battery, assuming I can find an official one with a decent remaining capacity or a non-shady refurbished battery outfit.
Time to break out the screwdrivers again...