The easiest is of course to just use the stateroom tonnage and cost for all such additional (luxury or other) features. Not easy on a small ship since there's not a lot of volume to work with. This is actually included in the first edition of HG (pg. 32) quoted below (note the * <see note below> and [text] is from my own notes):
"The following items are suggested uses for interior space aboard a starship. The actual masses and costs for such areas are left to the referee when actually designing such a ship. Many of these items are subsumed in the costs and tonnages of 4 ton staterooms. In most cases, such areas are required only when drawing up deck plans. Food Service Areas, including mess hall, galley, ward room. Scientific Areas, including commo suites*, avionics areas*, electronics counter-measures installations*, gunnery simulation trainers*, computer operations areas* and [ship maintenance and repair] parts storage*. Vehicle Decks, including garaging, maintenance bays, launching areas and parts storage. Recycling Stations*. Medical Areas, including isolation wards, surgeries, pharmacies, and examination rooms. Recreational Areas, including theatres, crafts shops, libraries, and pool rooms. Agricultural Areas, including fresh food gardens, hydroponics areas, and algae tanks. Troop Barracks, including squad areas, training rooms, armories, brigs, [small arms] ammunition magazines, vacc suit storage, capsule launch areas and briefing rooms."
Quite a bit to fit into the 2 tons of common space for each stateroom especially when you need corridors connecting it all
* I think some of the above, as noted by * in the text, is more appropriate to take out of the bridge tonnage which gives you a little more room to play with. Again don't forget half your bridge is probably the actual bridge with workstations for the crew and other electronics.