I agree with Kett to some extent in that the stock Vee is a well thought out bit of kit (255 moreso perhaps with engine and spec improvements) ...
Universally most would say seats must be changed to make the drive a little more engaging, apart from that the playing field is wide open - taking into account the limited 'off the shelf' options ...
Tuning and cosmetic bolt ons tend to be most common in the usual areas such as ...
Exhausts, Air Filter, Splitters, Spoilers etc.
Suspension modifications such as coilovers are quite common and also there are recent threads regarding brake alternatives ... Pad material is also an upgrade choice ...
Engine covers and bulkheads have got a lot of press and there are various options depending on tastes and requirements ...
Tuning is more specialist ranging from Chip upgrades on the OEM setup moving towards replacement ECU's such as Motec, and other associated upgrades around cams, headwork aswell as some examples of forced induction and Nitrous ... These require deeper pockets and include more risk ...
Wheels are less common but Trophy wheels have become popular but availability is low unless you are prepared to do research and look overseas.
I guess it's also worth mentioning Tyres as there is quite a bit of choice over the OEM Pilots so this is an option as tyres are pretty critical with the Vee ...
You also often find individual projects and options in particular areas have resulted in unique and diverse cars as owners come up with their own ideas and implementations rather than just buying stuff 'off the shelf' ...
If you have a Mk1 then options extend to upgrading to Mk2 stuff around the wheels, subframe, engine parts route but you have to ask the question as to whether the Mk2 option to start with is preferable ...
Much of this echos what has already been said and it's a question of personal choice and budget. The stock Mk2 is hard to beat IMHO but there are a lot of modificed cars also, no two are the same ...
Martin