I actually wouldn't find it too bad in a fallout shelter, except for the fact that you probably wouldn't have alot of air.
You use a filtration unit to draw in fresh air from outide. Fallout shelters don't need to be air-tight, you just need to make sure that all air coming IN has been thoroughly filtered (a series of high-quality modern HEPA filters woul probably do the job, actually), with an intake vent at least 3-5 feet off the ground. And maintain slightly positive pressure, so that the airflow through all other leaks is "from IN to OUT", thus keeping the radioactive dust outside your safe shelter.
And all the shelter would do would be giving you a few years of life in isolation, as you can never exit it due to the radiation.
"A few years" ...? Depending on how big it was, you could spend a
lifetime in a shelter.
Freeze-dried food has a shelf-life in excess of thirty years - so a twenty-year stay wouldn't necessarily be any especial hardship, foodwise. And for a lot of it, you only need to add water, and POOF, it's edible. Cold, but edible. Keep a camp stove and large supply of fuel on hand, and you can then heat that food up.
With enough storage space, you can also set aside a ton of candles, lamp oil, or even those "glow stick" things. Some of them are good for up to 12 hours apiece; they may not provide enough light to read by, but certainly, enough to walk around by.
Water is a problem - you can lay in a LOT of it, but you'd still need to go to outside sources eventually. There're several good ways to purify water, though (especially colloidal silver, which can also double as a medical antiseptic) - aside from radioactive particulates. Filtration might cover that angle, though.
For sanitation ...? Chemical Toilet. ^_^
Entertainment? Aforementioned power supply, a television and DVD player, and a ton of DVDs. Also, books. Especially RPG books, IMO. Boardgames galore - not just monopoly and such, but more-complex games like Civilization. And card games (the new acetate-film cards being highly recommended for their durability). If you have a sustainable model for power generation,
electronic format books become an option - nice, dense storage, there.
If it were me, I'd build a multilevel affair, VERY deep - the roof should IMO be at least 50' down. 100' would be preferable. Multi-layer enclosure around the shelter proper (alternating [rebar-embedded concrete] and [packed sand]) for ground-shock protection. Lay in materials for assembling a positive-pressure-enabled, airtight greenhouse on the surface after a few years (along with a solar-panel farm). And for assembling a few ground vehicles (multifuel-capable, preferably), in parts, as well. Low-tech agricultural and industrial stuff - and textbooks on their use. Medical supplies, and textbooks for that, too; the nearest doctor may be "
don't hold your breath" away ...!
Access would be via a series of stairs, climbing in a spiral pattern from a room BELOW the bottom floor of the shelter, with a grated floor, then a stairway back up to the shelter. This would provide composite blast protection (due to turning corners so often), and even some fallout-leak protection (let the dust settle to the bottom level, and through the grated floor, then you go back UP into the shelter). Also, you can use a series of open drains (or a septic-tank-like gravel leech field), and a supply of nonpotable water to hose down anyone returning from the exterior (to wash off any fallout stuck to your excursion outfit - a radiation or hazmat suit, if need be. Or just rubberised clothes, if surface radiation levels are low enough that you just want protection from tracking dust back inside.
150% as much sleeping space and consumables for your expected initial population - over the course of 20 years, after all, there may be a few births. Or you may just find your neighbors knocking on the door in a panic while the air-raid sirens are wailing, on Night 1. Having the capacity to let in a FEW more people is always a comforting throught, at the least. And ... there's always the possibility that you'll need to have stuff to TRADE with, so having excess is useful for that, too.
...
Now, if I can only find the $250,000,000 it would take for me to even contemplate building and equipping that sort of thing. (Seriously, once for giggles and gags, I priced just the FOOD supplies needed for 5 people over 20 years, plus some "tip of the iceberg" supplies for other stuff (enough glow-sticks to light a twelve-room shelter for ten years, 2 sticks per room per 12 hours) ... and it came to almost nine million dollars.
NOT including the actual shelter to put it all in!).