The best replacement for prepositional phrase is clause.

The best alternatives to prepositional phrase are rated as following:
★★★★★  clause
★★★★  infinitive phrase
★★★  adverb
★★  adverbial phrase
★  infinitive
★  adverb phrase
★  dependent clause
  participial phrase
  subordinate clause
  phrasal verb
  adverb clause
  indirect object
  verbs
  adverb prepositional phrase

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Speaking about prepositional phrase, one probably shouldn't forget about gerund.

Other search terms related to prepositional phrase and their ratings are:
★★★★★ gerund
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prepositional phrase
clause
infinitive phrase