To be honest, I've never counted the actual number of items per pack. So instead I'll give you my top three recommendations and what sorts of things they include. Seasons & Nightlife are my favourites, and you already have those. So here are my next favoured packs:
Open For Business
New things include shopping districts (like suburban downtowns), ownership of community lots, talent badges, lockable doors, becoming owners and employees of businesses, new abilities to earn, uniforms (that are buggy), business objects collection, work benches to create special objects (toys, flowers and robots), Servo the robot Sim, customised wedding clothes, children's lemonade stand, new and smaller dressers, display shelves, children and toddlers can change appearance, salon chair, new socials, new roofs, elevators, raised platforms, awnings, etc. As always, make sure you get the patch from here: http://thesims2.ea.com/update/?pid=Help_patches
Here's a list of the things that the University expansion pack introduced for the first time (some of these things were also included in later expansion packs): University education (obviously), Young Adult life stage, influence, 17 new socials, career outfits added to everyday wardrobe, lifetime wants, household merging, new foods (ramen and Chinese delivery), LAN-party computer playing, casual jobs, new parties (sports, toga and graduation), sports TV channel, additional wants and fear slots, new careers (Art, Paranormal, Natural Science and Show Business), new career rewards (camera, Resurrect-O-Nomitron, cow-plant and cosmetic surgery machine), zombies, career advancements, streaking, rapping (performing "freestyle"), new lots (including dorms, Greek Houses and Secret Societies), group music performances, hustling pool, counterfeit machine (new aspiration reward), new NPCs, multi-story build-mode objects (doors, arches and columns), connecting columns, ability to build walls on top of walls, and tons of new objects including a vanity on which you can makeover other Sims, a communal shower, instruments, fire sprinklers, coffee bars, cell phones, handheld games, MP3 players, etc. Get the patch!
New things include vacations (obviously), three vacation destinations, new collections and lots of new objects, beach ground covers, beach lots, pirate ship (largest object ever), Bigfoot, the dragon legend, voodoo dolls, sea chantey, vacation homes, campgrounds and tents, hotels, room service, changing booths now act as dressers, community lot marriage, undoing deaths on community lots, digging for treasure, beachcombing, new gestures to learn (bow, thump chest and hang loose), inviting foreign houseguests, jewellery (to buy and find), massages, mementos (special collectable experiences/memories), hula dancing, slap dancing, tai chi (for fun and fitness), new NPCs including the infamous Unsavoury Charlatan (pickpocket), the elusive Ninja and his teleporting secret, tea-drinking, axe-throwing, log-rolling, fire-dancing, sandcastles, sunbathing (and tans/burns), mahjong game, zen gardens (with meditative abilities), tree rings, shrines and temples, food stands, campfires, hot-springs, saunas, hammocks, new places to woohoo, new transportation picker (can travel to community lots with selected Sims), new turn ons/offs, pagoda roofs, new roof textures, honeymoons, secret vacation lots, pocket cameras for all Sims, souvenirs and display rack (with special influences with the right combinations), tours, post-vacation benefits, walking to community lots, etc. And, you guessed it, get the patch!