Firstly, I'm going to asume the bodyshell is sound, as there is no point doing anything if it is rotten (and Volvos of that age can rot very badly) - You'd end up spending hundreds on getting it welded back together...
I'll give an idea of prices in Sterling, obviously costs vary depending on where you are...
Creaks and groans are often a symptom of tired suspension. New dampers from companies like Monroe can be obtained cheaply, and will make a lot of difference. Budget for a couple of hours a corner if you're changing them yourself, less if a mechanic does it. The last set I got were £10 a corner off ebay.
Bushes will also be tired and worth changing. There are quite a few, and some will need a hydraulic press to change (which most decent garages should have). Polybushes from someone like Superflex are worth fitting if you intend to keep it a long time, and aren't usually much dearer than rubber ones. Budget around £5 per bush, plus fitting.
Hopefully you've been keeping these in good condition, if not then budget for new pads & shoes at least, as well as a fluid change.
The old Volvo redblock engines go on for ever, but make sure you keep up with all the routine maintenence, such as changing the timing belt, if it hasn't been done recently.
While you're at it, change the gearbox and diff oil, both will be pretty manky if they're original, and you'll probably find both will run a bit quieter with fresh oil.
Take the seats and carpet out and give them a good proper clean - getting rid of 20 years of built up grime will make it feel like a much better place to be! Tidy up or replace any broken trim, and give the paintwork a bit of attention (e.g. T-cut)
There is plenty more you could do, but I think sorting the suspension and tidying the appearance will make the biggest difference.