You could try a new induction kit for starters. I've got the Pipercross Viper which is said to give the best performance as it has a cold air feed attached so doesn't suck in hot engine bay gases. It's relatively quite compared to other kits on the market too.
Milltek cat back exhaust has a good build quality with a nice raspy note but not droning at motorway speeds.
Superchips Bluefin gives a nice crisp throttle response and revs a bit better at top end, especially above 4500rpm.
Denso Iridium plugs and Magnecor leads - not sure if they make much difference but I like the idea of performance plugs and leads.
Combine all the above and you'll be on your way to a bit more poke and adding the Janspeed manifold will certainly help further (next on the list ).
Or if you want to go the whole hog then a gas flowed head and cam from the likes of Lohen looks a good bet but obviously more expensive (one day I'll make that call )
Thats the engine sorted now the chassis.
GTT upper front brace
GTT lower front brace
GTT Mid brace
GTT upper rear brace
GTT spiroslot discs all round with Kevlar pads and Dot 5.1 fluid.
Eibach 30mm Lowering springs
That should keep it on the road nicely .
A lot of members replace the S spoke rims for lighter wheels and non runflat tyres with great results but personally I prefer the added safety and convenience of runflats, they saved me several times at motorway speeds from high speed punctures.
Above are all mods I've had carried out to my MINI and would certainly recommend and do again.
Oh and I get about 300-320 miles on a full tank of V-Power.
My Wifes got the sound kit fitted to her Cooper and its awesome. There's next to no power gain (about 3bhp?) but the improved sound of the exhaust and the airbox is brilliant It wounds better than my S with a stock exhaust. Also the remap that comes as part of the kit really does improve throttle response and removes some of the flat spots that you experience during acceleration and the exhaust note really sounds awesome at higher revs whilst giving a nice deep throaty growl on tickover.
Theres a sound kit register on here , you only have to read the owners comments to see the smiles it brings. http://www.mini2.com/forum/mini-cooper/114645-john-cooper-works-sound-kit-register-18.html