A 50 peaks (in the lakes, wales or scotland, at once or total for an individual in the year)
B 50 routes (in a day at a crag, for one climber or more)
C 50 climbers climbing at once
D 50 stars (day for the club, weekend for a team)
E 50 crags (climbing at 50 different crags during the year)
F 50 grades SPORT: f3, f4, f5, f5+, f6a, f6a+, f6b, f6b+, f6c, f6c+, f7a, f7a+...
UK TRAD mod, diff, vdiff, hvdiff, ms, s, mvs, vs, hvs, e1, e2, e3...
HUECO: v0, v1, v2, v3, v4, v5...
FONT: font3, font4, font5, font6a, font6b, font6c...
ALPINE: fac, pd, pd+, d, d+, ad...
SCRAMBLING: 1,2,3
DWS: s0, s1, s2...
AID (CLEAN): c1...
ICE: WI2, WI3, WI4, WI5...
SCOTTISH: I, II, III, IV... I reckon that's doable for us as a group?
G 50 miles walk, run, bike
H GPS trace making the shape "50". Or "L" for roman numerals...
I 50 sausages (BBQ theme)

Edit: other suggestions added below
J 50 km elevation gain walk, run, bike
K 50 indoor routes or boulder problems in a day
L 50 pitches full traverse of stanage
M 50 year old routes i.e. routes with a first ascent in 1965
N 50 outdoor/adventures films/lectures
O 50 mountains climbed
Or even 50 challenges!?*!
What do people think?